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 will use the IEFSSN00 parmlib member.
The order in which the subsystems are defined on the SSN parameter is the order in which they are initialized. For example, a specification of SSN=(13,Z5) would cause those subsystems defined in the IEFSSN13 parmlib member to be initialized first, followed by those subsystems defined in the IEFSSNZ5 parmlib member. you specify duplicate subsystem names in IEFSSNxx parmlib member, the system issues message IEE730. For more information, see the topic on handling errors in defining your subsystem in z/OS MVS Using the Subsystem Interface.