This message provides instructions for trade settlement. SettlInstSource (165) indicates if the settlement instructions belong to a broker or institution. It's been designed so that it can be sent between different types of institutions.
This message can be used in one of two modes (SettlInstMode (160)):
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.