A subsystem is a service provider that performs one function or many functions, but does nothing until it is requested. Although the term “subsystem” is used in other ways, in this section a subsystem must be the master subsystem or be defined to MVS™ in one of the following ways:

• Processing the IEFSSNxx parmlib member during IPL

  You can use either the keyword format or positional format of the IEFSSNxx parmlib member. IBM® recommends that you use the keyword format, which allows you to define and dynamically manage your subsystems.

• Issuing the IEFSSI macro
• Issuing the SETSSI system command

Note that the master subsystem (MSTR) is a part of MVS and is not defined in any of these ways. The following IBM subsystems use the SSI:

• JES2
• JES3
• IMS™
• Tivoli® NetView® for z/OS®

There are two types of subsystems:

• The primary subsystem. The job entry subsystem that MVS uses to do work. It can be either JES2 or JES3.
• Secondary subsystems. Secondary subsystems provide functions as needed by IBM products, vendor products, or the installation.