https://www.sec.gov/investment/settlement-cycle-small-entity-compliance-guide-15c6-1-15c6-2-204-2

In addition, shortening the standard settlement cycle to T 1 shortens the timeframe for broker-dealers, including those that are small entities, to comply with the requirement under Exchange Act Rule 10b-10 to give or send a written confirmation at or before completion of the transaction.  Specifically, Rule 10b-10 requires that a broker-dealer “give or send” the confirmation prior to settlement of a transaction; however, it does not require that the Rule 10b-10 confirmation be received prior to settlement.  In addition, the amendment to Rule 15c6-1(a) also will reduce the number of days that broker-dealers, including those that are small entities, will have to obtain possession of customer securities before being required to close out a customer transaction under Exchange Act Rule 15c3-3(m).

https://www.finra.org/rules-guidance/guidance/interpretations-financial-operational-rules/sea-rule-15c3-3-and-related-interpretations

15c3-3(m) Completion of sell orders on behalf of customers. If a broker or dealer executes a sell order of a customer (other than an order to execute a sale of securities which the seller does not own) and if for any reason whatever the broker or dealer has not obtained possession of the securities from the customer within 10 business days after the settlement date, the broker or dealer shall immediately thereafter close the transaction with the customer by purchasing securities of like kind and quantity: Provided, however, The term customer for the purpose of this paragraph (m) shall not include a broker or dealer who maintains an omnibus credit account with another broker or dealer in compliance with section 7(f) of Regulation T (12 CFR 220.7(f)).

Note to paragraph (m):See 38 FR 12103, May 9, 1973 for an order suspending indefinitely the operation of paragraph (m) as to sell orders for exempted securities (e.g., U.S. Government and municipal obligations).

Maybe someone who understands financial terms better can help decipher this, but it sounds like it might be saying that it might actually be a T 11 settlement date?