What this does is leaves DS1 in the concatenation alone. Replaces the DSN of DS02 w/the DSN of DS05, DS03 w/DS10, and DS04 w/DS20. It also makes the original DS05 a dummy dataset. The original DS06 thru DS20 are not processed because Dummy forces an immediate EOF on the 1st read.
If some of the params of DS05, 10 and 20 did not match those of DS02, 03 and 04, you may have to code those params in the overriding DD stmts. For example, if DS02 is not a cataloged ds and DS05 is, you have to code Vol=ser=,unit= in addition to the DSN param to nullify the volser and the unit params in DS02's DD stmt.
BTW, this is a highly esoteric ques for a job interview.
# 2) One way is xp compile both the programs and catch the main program in XP.Then come to the call stmt where it calls the sub program and change the program name on the top left corner to sub programe name.then press F9 so that it will come to the procedure division of sub program.
Another way is you dont have to catch the main program at all.
Instead you can intercep the sub program. connand is INT subprogram name.If you need more details on the second method i can help you.