When you add and define a subsystem, you make the subsystem's name known to the system. Previously, the only way to add a subsystem was to add and define it in the positional format IEFSSNxx parmlib member, which meant that an addition of a new subsystem required you to re-IPL the system.

You can still add a subsystem with the positional format IEFSSNxx parmlib member; however, you cannot use the dynamic SSI services if you add a subsystem this way.