z/OS supports multiple JES2 subsystems operating concurrently, with one subsystem designated as the primary subsystem and all others defined as secondary subsystems. A secondary JES2 does not have the same capabilities as the primary JES2 and some restrictions apply to its use. Most notably, TSO/E users can only access the primary subsystem. The restrictions are necessary to maintain the isolation from the z/OS-JES2 production system, but the convenience for testing is a valuable function. Operation of multiple copies of JES2 is referred to as poly-JES. Secondary JES2s can be useful in testing a new release or installation-written exit routines. This isolation prevents potential disruption to the primary JES2 and base control program necessary for normal installation production work.