Application programs request LU 6.2 services from VTAM® with the APPCCMD macroinstruction. The APPCCMD macroinstruction uses a request parameter list (RPL) to describe its requests to VTAM and to provide necessary processing information. The RPL is a control block that LU 6.2 application programs use frequently. It must be specified on each APPCCMD or other macroinstruction issued by the application to request VTAM services.

Application programs can explicitly set RPL fields themselves and specify only the address of the RPL on the VTAM macroinstruction, or they can use operands on the macroinstruction that cause VTAM to set the relevant RPL fields for a particular operation. As with any RPL-based request, the application program can specify an exit routine to receive control when the operation completes.

For APPCCMD requests, an RPL extension is used with the RPL. An RPL APPC extension supports only LU 6.2 functions. This extension is not appended to the RPL but is pointed to by the AAREA field in the RPL. In effect, the RPL for APPCCMD macroinstructions consists of a linked data structure. VTAM supplies the macroinstruction ISTRPL6 to create this extension and the DSECT ISTRPL6X to enable the application program to refer to its fields symbolically.