The company is evenly divided into thirds (eg: 100 man company - 1st third = 33, 2nd third = 33, etc..) Everyone in the company lists their MOS choices from 1-25(?). I think it's 25 total for men and 19 total for women (women cant select combat arms). Then, the SPCs/XO/CO go through and assign an MOS based on your standing and how they think you'll fit in the role. The thirds work in to give the Marine Corps a quality spread. The first person in the first third goes, then the first in the 2nd goes, then the first in the 3rd. So for the example company it would look like this for two rounds: 1-1, 1-2, 1-3, 2-2, 2-2, 2-3, 3-1, 3-2, 3-3. get it? It's based on 1) the needs of the Marine Corps 2) your standing and 3) your personal preferance. Sometimes the companies have SNA/SNFO slots to give out, sometimes they dont. To qualify for these slots (and thus, allow you to list 23 vice 21 MOSs), you MUST have a valid flight physical and qualifying ASTB scores. Without these, you can't list 7599 or 7580 on your choices. Depending on the company, 0302 is usually easy or hard to get. The MOSs with the largest numbers are usually things like Comm-O, Adj, Supply and the like. Combat Arms usually don't have that many slots save for infantry.
Clear as mud?