The application program sends or receives data in CSM buffers by issuing an APPCCMD send or receive macroinstruction with OPTCD=XBUFLST specified. The XBUFLST option indicates that the application is sending or receiving data using an extended buffer list. The extended buffer list points to data that resides in CSM buffers. See Macroinstructions used by HPDT applications for information about macroinstructions that send or receive data using HPDT.

The APPCCMD side of the HPDT interface also includes vectors that applications can use to determine whether a particular session supports HPDT services or to inform VTAM® about how the application intends to receive HPDT data. See Verifying the session's capabilities and Passing HPDT receive requirements to VTAM for more information about vector lists available to applications that use the HPDT interfaces.

The use of HPDT services on one end of a conversation is transparent to the partner LU. If an application specifies XBUFLST when the HPDT service is not available, VTAM executes the request using normal APPCCMD services.