If neither of these types of system base LU are defined in the active configuration, APPC/MVS rejects Allocate requests for outbound conversations from MVS programs (TSO/E users, started tasks, and other work requests) that are not associated with a scheduler or an LU. The Allocate callable service description, in z/OS MVS Programming: Writing Transaction Programs for APPC/MVS, explains which local LUs can be specified or used by default for outbound conversations.

MVS TPs that are scheduled in response to incoming allocate requests do not use a base LU. When an MVS TP receives an incoming allocate request, the request was sent over a session bound between two LUs, one of which was an MVS LU. When the MVS TP responds to the allocate request, it is automatically associated with the same MVS LU through which the request entered the system. Therefore, the only conversations that need a base LU are outbound conversations from programs not already associated with an LU.

When defining LUs with LUADD parameters in APPCPMxx parmlib members, it is advisable to designate one LU per transaction scheduler as the base LU. When more than one LU is defined as the base LU for a transaction scheduler, the most recently defined LU is the base LU.

If you are running APPC/MVS applications that expect the system base LU to be associated with a transaction scheduler, do not define a NOSCHED LU as the system base LU.