This has been difficult to find. It seemingly follows from Rule 15c3-3 . https://www.sec.gov/divisions/enforce/customer-protection-rule-initiative.shtml https:/

This has been difficult to find. It seemingly follows from Rule 15c3-3 . https://www.sec.gov/divisions/enforce/customer-protection-rule-initiative.shtml https://www.sec.gov/divisions/enforce/customer-protection-ru... But that's something of a slog to read. The closest I've found is footnote 13 in this rule change, which notes that it was requested to change this requirement. I haven't found anything contradictory. https://www.sec.gov/rules/sro/nscc/2013/34-69302.pdf https://www.sec.gov/rules/sro/nscc/2013/34-69302.pdf