The restart_nnn.XML files are created when both the ES_JES_RESTART and MF_UCC11 environment variables for ES are set to Y (these variables govern restart functionality) and a job has GDG datasets. These allow the correct GDG to be picked up on a restart of the job. The number after the restart_ is the job number it relates to.

The jobs will be restarted at the point in the JCL where there is RESTART= on the JOB statement, for instance:

RESTART=RUNIT1.JS020 

This means in the PROC RUNIT1 at jobstep JS020 the JCL would be restarted from there. However, keep in mind: after spool cleanup is run, all traces of the job are removed and it can no longer be restarted. These files can be deleted, if desired, but at that point a given job can not be restarted via the original JCL.