The difference between pax and cargo (or at least Fedex) is that although all the MEM based trips start in MEM, you may not fly a Fedex plane out of MEM (i.e. you deadhead commercially). FedEx wants to make sure a crew is at the city well ahead of time where the FedEx aircraft is located. That is why you have to be in position at a deadhead city 8 hours prior to show time. If for some reason you can't make it, they want to be able to find someone to fill the jet (either a reserve from MEM or as a last resort someone who lives in that city). You don't have to be in MEM 8 hours prior because there are usually plenty of reserves in MEM to fill a late sick call or other reason for a vacant trip.