What Is a Sub Customer? FINRA Margin, SIPC, and Tax Rules

A sub customer is the underlying account holder whose securities and cash are held at a carrying firm but who accesses the markets through an intermediary such as an introducing broker-dealer or investment adviser. The carrying firm clears trades, holds assets, and extends credit for that end client without dealing with them directly. Under FINRA and SEC rules, sub customers are often treated as distinct beneficial owners, which triggers specific obligations around margin, custody, reporting, and asset protection.¹

Who Ends Up as a Sub Customer

The label applies to several familiar market participants:

Retail clients of a non-clearing broker-dealer that routes trades through a larger firm for clearing and custody.
Overseas clients of a foreign broker-dealer that accesses U.S. markets through a domestic clearing firm.
Hedge funds, pension plans, and other institutional investors that reach the market through an intermediary’s master account rather than opening a direct relationship with the carrying firm.
Clients of a registered investment adviser whose accounts sit under a master arrangement at the carrying firm.

In each case, the person or entity holding the economic interest in the securities is one step, and sometimes more, removed from the firm that actually holds the assets.

How the Relationship Is Created

FINRA does not define “sub customer” in a single numbered rule. The concept comes out of Rule 4210, which defines “customer” broadly as any person for whom securities are held or carried and to whom a member extends credit, and Rule 4311, which governs carrying agreements between firms.¹²

Rule 4311 lets a carrying firm hold accounts on either a fully disclosed or an omnibus basis. In a fully disclosed arrangement, the carrying firm knows the identity of every underlying account holder, and each of those account holders is effectively a sub customer. In an omnibus arrangement, the carrying firm sees only the introducing firm’s single master account and may not know who the individual beneficial owners are unless a specific trigger requires disclosure.²

When one introducing firm acts as an intermediary for another introducing firm to obtain clearing services, sometimes called a piggyback arrangement, the firm must notify the carrying firm of the arrangement and disclose the other firm’s identity. That layering can put sub customers two or more steps away from the firm actually holding their assets.²

When Sub Accounts Must Be Treated Separately

Advisers and broker-dealers often open a single master account with a carrying firm and split it into sub accounts for individual clients or strategies. FINRA Regulatory Notice 10-18 sets the core rule: if the sub accounts identify the beneficial owner of each sub account and the carrying firm knows those identities, the firm must recognize each sub account as a separate customer account for purposes of FINRA rules and federal securities laws.³

That recognition matters because it drives margin calculations, SIPC coverage, and reporting.

Two exceptions allow beneficial-owner identities to stay undisclosed to the carrying firm:

A registered investment adviser acting in a bona fide capacity may use sub accounts for each client it advises without revealing each client’s identity to the carrying firm.
A registered introducing broker-dealer may obtain clearing services on an omnibus basis without disclosing the identities of its individual customer accounts.

Even under those exceptions, a carrying firm cannot ignore warning signs. If it has actual notice, or encounters red flags suggesting different beneficial owners behind the sub accounts, it must investigate. Red flags include sub accounts receiving separate trade confirmations or reports, being charged separate commissions, or being documented independently from the master account. Once the firm learns who the beneficial owners are, it has to treat those sub accounts as separate customer accounts going forward.³

Margin Rules That Apply to Sub Customers

Sub customers get the same margin treatment as any other customer under FINRA Rule 4210. The carrying firm has to collect and maintain collateral meeting the rule’s percentage thresholds for every position, regardless of the account coming in through an intermediary.¹

For long equity positions, maintenance margin is 25 percent of the current market value. If a sub customer’s account equity falls below that, the carrying firm has to call for more funds or securities. Any deficiency must be resolved as promptly as possible and no later than 15 business days from when it occurred, unless FINRA grants more time.¹

Carrying firms also have to review whether specific securities or accounts warrant higher requirements than the rule’s minimums. That review matters more for sub customer accounts, where the carrying firm has less day-to-day visibility into how the intermediary’s clients are trading. Concentrated positions, thinly traded stocks, and volatile accounts can all draw higher requirements.¹

Some sub customer accounts face tighter deadlines. Portfolio margin accounts, which use risk-based modeling, allow only three business days to deposit funds or establish a hedge before the carrying firm has to begin liquidating positions to cover a shortfall.¹ Pattern day trader accounts must maintain at least $25,000 in equity under the current rules.

How Sub Customer Assets Are Protected

SEC Rule 15c3-3, the Customer Protection Rule, requires carrying firms to segregate customer assets from their own proprietary holdings and maintain a cash reserve for the benefit of customers. The rule defines “customer” broadly as any person from whom or on whose behalf a broker-dealer holds funds or securities.⁴

The rule generally excludes other broker-dealers from that definition but carves out an important exception: a broker-dealer maintaining an omnibus account for its own customers at a carrying firm is treated as a customer to the extent of that omnibus account. The carrying firm must include those omnibus assets in its reserve calculations, protecting the sub customers’ holdings even when the carrying firm does not know their individual identities.⁴

The practical effect is that sub customer assets held by a carrying firm are ring-fenced. If the carrying firm gets into financial trouble, customer securities held in segregation and cash in the reserve account are not available to the firm’s general creditors.

SIPC Coverage

If a carrying firm fails and enters liquidation under the Securities Investor Protection Act, SIPC covers each customer’s account up to $500,000 in total, including a $250,000 limit for cash claims. The $250,000 cash advance limit has been confirmed to remain at that level through at least January 2032.⁵

How that coverage flows to sub customers depends on the account structure. SIPC determines coverage by separate capacity: each distinct capacity in which a person holds an account receives its own $500,000 of protection. When sub accounts are treated as separate customer accounts because beneficial ownership has been identified and disclosed to the carrying firm, each sub customer generally qualifies for separate SIPC coverage. When accounts are carried on an omnibus basis without disclosed beneficial ownership, the picture is more complex and may not provide individual protection to each underlying account holder.

Tax Reporting on Sub Customer Income

The IRS assigns Form 1099 reporting responsibility based on whether the intermediary between the carrying firm and the sub customer is a qualified or nonqualified intermediary, a distinction that matters most for foreign intermediaries.⁶

When the intermediary is a nonqualified intermediary, the IRS treats the sub customers themselves as the payees of any income. The carrying firm, or whichever entity is the withholding agent, must issue Forms 1099 as if it were paying the sub customers directly.⁶

A qualified intermediary may assume primary 1099 reporting and backup withholding responsibility. If it does, the carrying firm treats the qualified intermediary as the payee and does not report on each sub customer individually. Whether the qualified intermediary has assumed that responsibility is determined from its Form W-8IMY. If it has not, the carrying firm has to look through to the sub customers and issue 1099s to each U.S. person the intermediary acted on behalf of.⁶

Identity and Documentation the Carrying Firm Must Keep

Opening a sub customer account requires the carrying firm to verify the identity of the underlying beneficial owner wherever that identity is disclosed. That means collecting tax identification numbers, confirming the intermediary’s regulatory registration, and keeping records of the carrying agreement itself. Those records let the firm meet anti-money-laundering and Know Your Customer obligations.²

For sub customers that are legal entities such as funds, corporations, or trusts, the carrying firm must also identify beneficial owners with significant control. Under 31 C.F.R. § 1010.230, a beneficial owner includes any individual with significant responsibility to control, manage, or direct the entity, such as an executive officer or senior manager. A 2026 FinCEN order gave financial institutions some relief: rather than re-verifying beneficial ownership at every new account opening, firms may rely on previously obtained information as long as the customer certifies it remains accurate and the firm has no reason to question its reliability.⁷

If the firm learns facts that reasonably call into question the accuracy of the previously obtained information, it must re-verify the identities of the entity’s beneficial owners before proceeding. Those records form the audit trail FINRA and SEC examiners review during routine inspections of both the carrying firm and the intermediary.⁷

1
FINRA. 4210. Margin Requirements
2
FINRA. 4311. Carrying Agreements
3
SEC. FINRA Regulatory Notice 10-18 – Master Accounts and Sub Accounts
4
eCFR. 17 CFR 240.15c3-3 Customer Protection – Reserves and Custody of Securities
5
SIPC. What SIPC Protects
6
Internal Revenue Service. Foreign Intermediaries
7
FinCEN. Exceptive Relief from Requirement to Identify and Verify Beneficial Owners at Each Account Opening FIN-2026-R001