The Free On-line Dictionary of Computing (30 December 2018):

Job Entry System

    (JES) An IBM mainframe term.  There are
   really two JESs.  JES2 is smaller and simpler, and can handle
   99.99% of most jobs that run on IBM's MVS operating
   system.  JES3 is much bigger and requires really big iron
   to run.

   (1996-03-23)