ACP 128 (ALTERS): Message Format, Precedence, and Routing

ACP 128 is the allied military communications standard that governs how official record messages are formatted, prioritized, routed, and delivered between coalition nations. Its full title is Allied Communication Publication 128, Allied Telecommunications Record System (ALTERS) Operating Procedures. The most recent publicly cataloged version is Edition (B), published in April 2016.¹ It exists so that an Australian, British, Canadian, New Zealand, or American operator can send a message and have every other nation’s system parse it identically, with no translation layer in between.

Who Publishes ACP 128 and Who Uses It

ACP 128 is issued under the authority of the Combined Communications-Electronics Board (CCEB), whose member nations are Australia, Canada, New Zealand, the United Kingdom, and the United States. The Board sets the content, format, and release policy of all Allied Communications Publications, and ACPs are published by the United States after unanimous agreement by the CCEB members.²

Before publication, each ACP is staffed through NATO, so NATO nations beyond the core five also adopt and operate under these procedures. ACP 128 sits within a larger family: ACP 117 governs routing indicator assignments, ACP 121 covers communications system doctrine, ACP 126 addresses tape relay procedures, and ACP 130 defines visual signaling.

How an ACP 128 Message Is Built

Every ACP 128 message has four parts: the Heading, the Address, the Text, and the Ending. The Heading carries routing and transmission data. The Address identifies who must act on the message and who receives it for awareness. The Text is the content. The Ending closes the transmission with validation data confirming it arrived intact.

Underneath those four parts, the message is organized into numbered format lines, each carrying a specific type of data at fixed character positions. The operationally important ones are:

Format Line 1 contains the start-of-message indicators, transmission identification, and any pilot characters needed to synchronize equipment.
Format Line 2 is the densest line in the message. It packs the precedence designator, classification, the originating station’s routing indicator, the station serial number, the filing date and time, a classification redundancy check, and the routing indicators for all destination stations.³
Format Line 4 carries transmission instructions, including the security warning operating signal (ZNR for unclassified, ZNY for classified) and any special operating groups directing non-obvious handling.
Format Line 5 holds the precedence prosign, the Date-Time Group, and any operating signals conveying additional handling instructions.
Format Lines 6, 7, and 8 identify the originator (prosign FM), the action addressees (prosign TO), and the information addressees (prosign INFO).³
Format Line 9 lists exempt addressees (prosign XMT), used when a collective address group includes recipients who should not receive that particular message.
Format Line 12 contains the full security classification, any codewords, caveats, compartments, and handling instructions.⁴

The prosign BT (long break) marks the boundary between the text and the surrounding parts of the message, appearing immediately before and after the text body.⁵ The prosign AR signals the end of the entire transmission.

The Date-Time Group

The Date-Time Group (DTG) records when the message was released or handed to a communications facility. Format is six digits followed by a time zone suffix, a three-letter month, and a two-digit year. The first two digits are the day, the next two the hour on a 24-hour clock, and the last two the minutes. A message released at 12:30 PM Greenwich Mean Time on July 18, 1995, carries the DTG “181230Z JUL 95.”⁶ The suffix “Z” (Zulu, Greenwich Mean Time) is used for virtually all record traffic; local zones are reserved for tactical situations.

Precedence and Speed of Service

Every ACP 128 message carries a precedence designator that dictates how fast it must move. The four operational categories, lowest to highest urgency, are:

ROUTINE (R): standard traffic with no special urgency, processed in the order received after higher-precedence messages have cleared.
PRIORITY (P): traffic requiring prompt action or supporting operations already in progress. Priority messages should be delivered immediately on receipt at the destination, and routine traffic under way should not ordinarily be interrupted for them unless a substantial portion of the routine message remains unsent.⁷
IMMEDIATE (O): reserved for situations gravely affecting national security, force safety, or operations requiring immediate delivery. Lower precedence messages are interrupted on all involved circuits until IMMEDIATE traffic clears.⁷
FLASH (Z): the highest operational precedence, used for initial enemy contact reports or messages of extreme urgency. Flash traffic is hand-carried, processed, transmitted, and delivered ahead of everything else, interrupting lower-precedence traffic on all circuits until complete.⁷

A fifth category, FLASH OVERRIDE (Y), sits above FLASH. It is reserved for government-level strategic communications and is not used by military operational units. When a FLASH OVERRIDE message enters the system, it takes precedence over everything, including FLASH traffic.

The Department of Defense has historically defined end-to-end speed-of-service targets measuring time from receipt at the originating communications center to readiness for delivery at the destination: FLASH within 10 minutes, IMMEDIATE within 30 minutes, PRIORITY within 3 hours, and ROUTINE within 6 hours.⁸ Modern automated systems routinely beat those numbers, but the hierarchy still governs queuing: a FLASH message always displaces IMMEDIATE traffic, and so on down.

Routing Indicators and Address Groups

Routing indicators tell the ALTERS network where to send a message. Each is a group of alphabetic characters assigned to a specific station. In Format Line 2, the originating station’s routing indicator sits in a fixed seven-character field, and the destination indicators follow after the classification redundancy check.³ A routing indicator identifies the receiving station and can also encode its geographical area and operational status. Detailed composition rules are maintained in ACP 117 and ACP 121.

When a message needs to reach many recipients, the originator uses an Address Indicating Group (AIG) instead of listing every station. An AIG is a pre-assigned designator, formatted as the letters “AIG” followed by an identifying number, that represents a defined distribution list of military and civilian addressees. Only military facilities may originate messages addressed to AIGs.⁹

Individual stations can also be addressed by their Plain Language Address Designator (PLAD), the telegraphic name assigned to a U.S. Government transmission facility and listed in ACP 117.⁹ The distinction matters: routing indicators move the message through relay infrastructure, while plain language addresses and AIGs tell the final communications center who needs to read it.

Service Messages and Error Handling

ACP 128 builds in a structured toolkit for catching and correcting problems. The primary mechanism is the Z-code system: short alphanumeric signals that operators and automated systems use to report errors, request retransmissions, and confirm receipt quality.

ZNR / ZNY: the security warning operating signals in Format Line 4. ZNR marks unclassified traffic; ZNY warns that the message is classified.
ZES: tells the originator that a message arrived incomplete or garbled and requests retransmission.
ZBK: a quality check asking whether the receiving station is getting clear traffic or garbled signals.
ZAH: reports that a message cannot be relayed in its present form because the format is incorrect, prompting the originator to send a corrected version.
ZWN: sends the corrected version of a portion of a previously transmitted message that contained errors.

When a message fails to arrive at all, the intended recipient’s serving communications center can initiate a tracer. Under U.S. Army procedures, a tracer is filed using a standardized non-receipt claim form, and tracer action must be initiated within 30 days of the original transmission date, because communications centers retain incoming and outgoing messages for 30 days.¹⁰ After that window, the trail is cold.

Security Built Into the Format

Security is not bolted on. The classification designator sits at a fixed position in Format Line 2 and is repeated as a redundancy check. Format Line 4 opens with the security warning operating signal. Format Line 12 carries the full security marking, including classification level, compartment codewords, caveats, and any special handling instructions.

Automated processing systems perform input and output security checks by scanning Format Line 12 and comparing every classification element against a table of what is authorized for the circuit carrying the message. Any mismatch pulls the message from automated routing and places it on a supervisor queue for manual review.⁴ This prevents classified traffic from spilling onto a circuit not cleared for that level.

Retention Windows

Every message processed through ALTERS generates an audit trail. Communications facilities log the message count, receipt or transmission time, originating station, message size, and security characteristics.⁴ Each successfully transmitted or received message is recorded with the originating station routing indicator, station serial number, DTG, and channel designator and sequence numbers.

U.S. Army policy requires telecommunications centers to keep copies of incoming and outgoing messages for 30 days. Originators of organizational email sent through military record systems must also retain their copies for 30 days from the original transmission date. For privacy communications, reference copies are held for a maximum of 30 days and then destroyed, while the authorized user is responsible for maintaining their own record files under separate records-management regulations.¹⁰ Other nations operating under ACP 128 follow their own national supplements for retention timelines.

Why ACP 128 Still Matters

ACP 128 was originally written for teletypewriter circuits and physical tape relay. The equipment is long gone. The standard has aged better than that history suggests. Strict message format, precedence hierarchy, security architecture, and error-correction protocols all translated cleanly into digital store-and-forward systems. Modern military messaging platforms still enforce the same format line structure and precedence queuing, even though messages now move as data packets rather than paper tape.

The interoperability problem is what keeps ACP 128 in service. When Australian, British, Canadian, New Zealand, and American forces need to exchange formal record communications during a joint operation, they all parse the same format lines, apply the same precedence rules, and use the same Z-codes to flag problems. Edition (B), published in April 2016, updates the publication for digital systems while preserving that shared grammar.¹

1
Standards Central. ACP-128 Free Access via Standards Central
2
CIA FOIA Reading Room. Military Communications Electronics Board
3
Electronics and Books. Allied Telecommunications Record System (ALTERS) Operating Procedures
4
CIA FOIA Reading Room. Communications Support Processor Functional Description Manual
5
Communications Museum. ACP 130 Communications Instructions Signalling Procedures in the Visual Medium
6
Scribd. ACP 128 A
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Navy Radio. SSTS 56001F Message Preparation and Handling
8
UNT Digital Library. Study of Automation of Message Handling Functions at USCG COMSTA RADSTA and Communication Centers
9
U.S. Department of State. 5 FAH-2 H-230 Telegraphic Address and Precedences
10
Federation of American Scientists. Army Regulation 25-11 Information Management Telecommunications Record Communications and the Privacy Communications System