Each airline's pass travel system works a little differently. With Delta's, your boarding priority is based on what your pass priority is (S3, S3B, S4) AND what the employee's date of hire is. So if you're traveling as a S3, and your employee's date of hire is, say, 10/21/99, and another S3 shows up who's employee's date of hire is 11/05/95, they are 'senior' and will get on the plane first. American's system is different, you still have different pass priorities, such as a D1, D2, or D3, a D1 will always get on before a D2, and so forth, but if you as a D2 show up for your flight 2 hours before departure, you will get on the plane before any D2 who shows up after you, it's based on time of check-in, and you can check in at the airport for your flight up to 4 hours in advance of the departure time. Both systems have their good and bad points, and I really don't think one is 'better' than the other, they're just different.
Just a note, pilot jumpseat priority can differ also, at Delta, a pilot jumpseat is given first come first serve, and a pilot can call and actually 'reserve' the jumpseat for a particular flight. At American, a pilot is not assigned the jumpseat unless all the seats in the back are full. Then, the jumpseat is given to the pilot with the highest senority, regardless of check-in time. Also, if said jumpseat is on an AA aircraft, any AA pilot has priority to that jumpseat before an Eagle pilot, or a pilot from another airline. Conversely, if said jumpseat is on an Eagle aircraft, any Eagle pilot has priority on that jumpseat over an AA pilot, or a pilot from another airline.
It's confusing at first, but you learn as you go. Especially if you're a commuter, you get used to the system real quick! Last summer commuting between IND and LGA/JFK I used both my husband's travel card on Delta (ACA had 3 flights a day direct between IND and LGA) and my travel card on AA (I had to go from IND to ORD or sometimes even DFW to get to LGA). Standby travel can be tricky, especially if you're traveling with kids, or a big group. When people would ask me if I could give them cheap tickets, I would tell them to buy a ticket online. You get just as good, if not better a price, a guaranteed seat, and a lot less headache. Most airlines have dress codes for their non-rev passengers, and some gate agents have been known to be real sticklers. As a non-rev, you usually can't wear shorts, T-shirts, sweatshirts, beach wear (including flip-flops), and if you're sitting in first class the dress rules are even more stringent. (Often no denim, and no sneakers.) Also, as a non-rev your behavior reflects on the employee! At AA I knew a few people who had their own travel benifits recinded because of how someone acted while riding on their passes! Apparantly in at least one case the person using the pass didn't understand the concept of 'standby' and threw a hissy fit when the flight was full and they didn't get on, they created such a scene that the employee lost their travel card. Forever.