Subsystem initialization is the process of readying a subsystem for use in the system. IEFSSNxx members of SYS1.PARMLIB contain the definitions for the primary subsystems such as JES2 or JES3, and the secondary subsystems such as NetView® and DB2®.

During system initialization, the defined subsystems are initialized. You should define the primary subsystem (JES) first because other subsystems, such as DB2, require the services of the primary subsystem in their initialization routines. Problems can occur if subsystems that use the subsystem affinity service in their initialization routines are initialized before the primary subsystem. After the primary JES is initialized, the subsystems are initialized in the order in which the IEFSSNxx PARMLIB members are specified by the SSN parameter. For example, for SSN=(aa,bb), PARMLIB member IEFSSNaa would be processed before IEFSSNbb.