The SYSTEM option generally effects only the way parameters are passed to MAIN. The default is SYSTEM(MVS), and this option should be used for all programs except as noted below.
The SYSTEM(CICS) option should be used for all CICS MAIN programs.
The SYSTEM(IMS) option should be used only for those IMS MAIN programs to which IMS will pass parameters BYVALUE.
The SYSTEM(OS) option should be used only for those z/OS UNIX MAIN programs that want to receive the parameter list built by z/OS UNIX. For more discussion of this option, see the Enterprise PL/I for z/OS Programming Guide.