It converts the 7-byte PD input value starting in position 9 to an output value of 'sdd...d' starting in position 1, where s is + for a positive value or - for a negative value and d is 0-9. So P'1234567890123' would be converted to '+1234567890123' and P'-1' would be converted to '-0000000000001'.
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: