Message Type Identifies the type of message Message Length Specifies the cumulative length in octets of the Message ID, Mandatory Parameters, and Optional Parameters. Mandatory Parameters Variable length set of required message parameters. Some messages have no required parameters. For messages that have required parameters, the required parameters MUST appear in the order specified by the individual message specifications in the sections that follow. Optional Parameters Variable length set of optional message parameters.
Many messages have no optional parameters.
For messages that have optional parameters, the optional parameters may appear in any order.