MSRB Rule G-12 essentially governs the manner in which broker-dealers confirm or compare trades with each other and settle deliveries. The FINRA analog is the Rule 11000 Series ("Uniform Practice Code"). See also SEA Rule 15c6-1 ("Settlement Cycle").
MSRB Rule G-15 governs trade settlement and the sending of confirmations to customers.
See also the topic page on Muni Trade Confirmations as it pertains to confirmations to customers.