Well, I will share my experience at usairways when I was an agent. We had three basic types of travel status: NRSA, NRSP, and MR (must ride).

Let's break them down individually:
*NRSA*
This is what your status was if you were using your "benefit" of being able to fly for free. If you were a flight crew member, this is what you listed under to commute to base. If there is a seat, then you go. I worked for usair as the merger with HP was going through. Their priority for NRSA was based on check in time. At us, we flew by seniority. Here was the breakdown of priority codes:
SA1--(highest priority code... We got three PNRs a year to list at this status--you trump all other non-revs, except for those more senior flying SA1. Basically, everyone used these on routes to Europe/Hawaii)
SA2--a special code for an employee who needed a little help/compassion from the airline. It required approval from the pass office, but it was basically if you were traveling to a funeral. You could request and have more priority.
SA3-- this was the general employee priority code. It was the status for all active employees of usairways and all wholly owned subsidiaries. (Including psa and piedmont). Fly by senority and accompanying family/guest passes also flew sa3.
SA4--family members of sa3 riders flying alone and retirees. Senority sorted.
SA5--active employees of regional airlines associated with US. Senority based. ***caveat*** a ZW employee on the list would trump all other fliers on a ZW flight. (Same was true for all other regionals, not wholly owned)
SA6--unaccompanied family of sa5 riders.
SA7--buddy passes. Sorted by check in time.
SA9--- other airline employees flying as a jumpseat, zed fare, station pass, etc. sorted by check in time.

*NRSP*
This was used when you were flying on official company business. It required a ticket to be issued (as opposed to auto electronic as a pass rider). You were confirmed with a seat, but COULD be removed if necessary for oversales, weight and balance, etc.

The way we thought of it is if any company employee was going to be on the plane, it would be the space positives.

*MR*
These are must rides. They are employees traveling on a flight and cannot be removed. A revenue passenger would be idb before a MR was taken off. These were pilots and flight attendants who were deadheading to work from a different base. Occasionally, mainline pilot and flight attendant commuters could get listed as a MR if they had listed for two flights previously and couldn't get on. (Contract provision/protection). MR employees were guaranteed a cabin seat. Space positive was only guaranteed a seat (could be jumpseat if necessary).

That's how the cookie crumbled at US.