The Settlement Instructions <T> message provides either the broker's or the institution's instructions for trade settlement. The SettlInstSource <165> field indicates if the settlement instructions are the broker's or the institution's. This message has been designed so that it can be sent from the broker to the institution, from the institution to the broker, or from either to an independent "standing instructions" database or matching system.
The Settlement Instruction detail can be either explicitly specified (via SecuritySettl* and CashSettl* fields) or can exist on within an independent standing instructions database and can be referenced via the StandInstDbType <169>, StandInstDbName <170>, and StandInstDbID <171> fields.
See Appendix F - Settlement Instructions <T> Field Usage Matrix