It avoids a SUM overflow by reformatting the 3-byte ZD field in positions 4-6 into a 5-byte ZD field in positions 4-8. The INREC statement adds 2 binary zeros before the 3-byte ZD field to make it a 5-byte ZD field. It probably should be adding two character zeros (C'00' = X'F0F0') since it's a ZD field, but X'0000' will work since the zone is ignored. (Perhaps whoever created the job thought 2Z meant 2 ZD zeros instead of 2 binary zeros.)
More specifically:
INREC - Creates a record with the following fields:
1-3 - input positions 1-3
4-5 - binary zeros (X'0000')
6-8 - input positions 4-6
SORT - sorts on positions 1-3
SUM - for records which have the same value in positions 1-3, keeps one of the records and sums the values in positions 4-8 (e.g. X'0000F1F2F3).
OUTREC - Creates an output record with the following fields:
1-3 - input positions 1-3
4-9 - the ZD total in positions 4-8 edited to a displayable value of dd,ddd with leading zeros suppressed, e.g.