Deciding under which subsystem a started task should run

To decide under which subsystem your started task should run, determine what services the task requires and what support the primary (job entry) subsystem, the master subsystem, and other subsystems provide.

Inform the system programmer responsible for the master JCL of your decision. Then code the name of the subsystem on the START command's SUB= keyword.

Without a SUB= keyword on the START command, the operating system will create the started task under the primary job entry subsystem (JES2 or JES3) unless the task itself is a subsystem, that is, it is either defined

in the member IEFSSNxx of SYS1.PARMLIB, or
dynamically by the SETSSI command or IEFSSI macro.

(A subsystem, unless requested to start under the primary JES subsystem by setting flag SSCTUPSS in the SSCVT, starts under the master subsystem, MSTR.)

A started task, regardless of the subsystem under which it runs, is demand-selected and runs in its own address space. Several considerations apply:

The task can be a multi-step procedure or a job.
It may not use operating system restart facilities. (The system does not support step restarts or checkpoint restarts for started tasks.)
The JCL for the started task may contain the following statements:
- COMMAND
- ELSE
- ENDIF
- IF/THEN
- INCLUDE
- SET
The system defines the system symbolic parameter &SYSUID. If the member name that is the target of the START command matches an entry in the started procedures table, &SYSUID contains the corresponding userid from that table. Otherwise, &SYSUID contains a null string. For information on the started procedures table, see z/OS Security Server RACF System Programmer's Guide.