The SSN parameter in IEASYSxx identifies the IEFSSNxx member that the system is to use to initialize the subsystems, as follows:
SSN={aa }
{(aa,bb,...)}The two-character identifier, represented by aa (or bb, and so forth) is appended to IEFSSN to identify IEFSSNxx members of parmlib. If the SSN parameter is not specified, the system uses the IEFSSN00 parmlib member.
The order in which the subsystems are specified on the SSN parameter is the order in which they are defined. For example, a specification of SSN=(13,Z5) would cause those subsystems specified in the IEFSSN13 parmlib member to be defined first, followed by those subsystems specified in the IEFSSNZ5 parmlib member. If you specify duplicate subsystem names in IEFSSNxx parmlib members, the system issues message IEFJ003I to the SYSLOG, the INTIDS console, and consoles that monitor routing code 10 messages.
Some exits that use system services may run before other system address spaces are active. You must ensure that any address spaces required by the system services are available prior to invoking the service.
For more information, see the topic on handling errors in defining your subsystem in z/OS MVS Using the Subsystem Interface.