Pos1-->the pos of the out record
Pos2-->the pos from the input record
length--> the number of bytes selected from the input record from Pos2
p, m,CHANGE=(v,find,set,...) to give the position and length of the input field, the length of the output field, and the ″table″ consisting of pairs of find constants, and set constants or set fields.
NOMATCH=(set) to set the output value when an input field value does not match any of the find constants.
for your statement,It will select the first 6 bytes from the input as the output record, and l change 'U' to 'C' and 'D' to 'C' in the position 6 of the input file, for others, just remain the same.
If you're not familiar with DFSORT and DFSORT's ICETOOL, I'd suggest reading through "z/OS DFSORT: Getting Started". It's an excellent tutorial, with lots of examples, that will show you how to use DFSORT, DFSORT's ICETOOL and DFSORT Symbols. You can access it online, along with all of the other DFSORT books, from: