A couple of credit items/what it's used for:
-Within a trip/pairing, credit is anything above block (this is where your rigs and DH pay is indicated)(This is what the sodomizer is minimizing. According to the company, credit is bad, bad, bad. Paying pilots when they aren't operating/creating revenue)

-For the current month: Credit is used to determine your individual pickup limit. (ALV+15). It's also used to determine pay-treatment for GS (must meet GS trigger)

-For PBS: used to determine what a pairing is worth for line construction. You'll see 'in-period credit' when you have a carry-out trip. Your line must fit within the 'line credit window' which I believe is pretty well covered in the SRH/PBS references.
-Also for PBS: Vacation and training events all have associated credit values. PBS uses these values during the line construction process. (highly recommend John Bell's Reserve Calculator on the PBS page of DLnet and linked from the PBS webapp.)