The USERFLD field of the RPL extension enables the application program to uniquely identify an RPL. This field could contain the address of a data structure that represents a conversation. With the RPL extension, the application program can handle a series of input and output actions for a particular conversation. For example, the application program might associate a conversation request with the address of a user-defined data structure that contains information about the conversation that the application program needs.
The USERFLD field contains space for 4 bytes of information. VTAM® saves the information that is placed in the field at the time a conversation is established. This information is available to the application program in the RPL extension specified for a particular macroinstruction.