When an APPCCMD CONTROL=ALLOC, CONTROL=PREALLOC, or CONTROL=RCVFMH5 completes successfully, VTAM® places a session identifier in the SESSID field of the RPL extension (RPL6SSID in the ISTRPL6X DSECT). This session identifier uniquely identifies the session being used by the conversation. The PLU is responsible for generating the SESSID. When VTAM generates this SESSID, it is derived from the procedure-correlation identifier (PCID).

The session identifier may be the same as the session ID displayed by the DISPLAY NET,SESSIONS VTAM operator command. This case is true if the bind request contains a session instance identifier structure subfield with a format byte equal to X'01' and the corresponding BIND response contains a session instance identifier structure subfield with a format byte equal to X'02'. If the retired format bytes (X'00' in the BIND request, X'00' or X'F0' in the BIND response) are present in the session instance identifier subfield, then the session identifier will not be the same as that displayed by the D NET,SESSIONS VTAM operator command.

The identifier ranges from 2 to 8 bytes in length. The length of the session identifier is returned in SESSIDL (RPL6SIDL in the DSECT).

An APPCCMD CONTROL=RCVFMH5 macroinstruction can complete successfully without having a session identifier returned (SESSIDL=0). This is the case when the use of an associated session for a pending conversation is complete, and the session has been released for use by another conversation.