Which of the following is NOT required to be recorded on customer accounts prior to settlement under MSRB rules?

The requirement for recording information on customer accounts prior to settlement under MSRB (Municipal Securities Rulemaking Board) rules emphasizes the importance of certain identifying and regulatory information to promote transparency and compliance. The specific elements that must be recorded include the customer’s name, address, employer details, and the signature of a municipal securities principal.

However, while obtaining a customer's social security number can be essential for identification purposes and is often collected, it is not explicitly mandated by MSRB rules to be recorded before settlement. This distinction is essential to note because the social security number is generally more related to tax identification rather than an absolute requirement for customer account setup or transactions in municipal securities.

This understanding illustrates that while collecting robust customer identifiers is crucial for effective risk management and regulatory compliance, certain pieces of information, like the social security number, may not be legally required to be documented in the context specified, thereby making it the correct choice in this scenario.