control level field & Z-add

What does the control level field in
the cspec prompt do for a subroutine in
rpg/400? i've done several rpg/400
pgms w/subroutines & never once remember
adding an SR in that field. what gives?

also what would this line do at the
end of a clrsfl subroutine:

 Z-ADD 0  RRN  4 0

i'm gathering it clears a 4 digit rrn
with 4 zeros when the clrsfl runs.

regards,

Mohammad
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