1) When you use PGM=ICEMAN or EP=ICEMAN, you invoke the sort product installed at your site which may be DFSORT (ICExxxs messages), Syncsort (WERxxxs messages) or CA-Sort (CASxxxs messages). Alternatively, you can use PGM=SORT or EP=SORT.
2) and 3) 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: