Billets are indeed jobs within the det. I can't speak for NROTC, but I would assume it is like AFROTC. There are certain jobs for certain ranks, as you progress in ROTC then other positions will open up to you. In other words it mimics actual AD military life. You are not going to see a C100 be the commander of cadets, that position is held for C400 cadets.
At our DS's det., they rotate the cadets every semester, so just because he has a billet this semester does not mean he will have it next semester. They do this because there are limited amount of jobs for the cadets and they want to spread the wealth. This also depends on the size of the det., obviously smaller dets may not follow this scenario, but larger ones usually do. I say that so cadets who did not get a position this fall, don't think that they have now been labeled as behind the power curve. It might just be that the det is so large, at this time they can't offer a position, but they most likely will the next semester or the semester after that.
The important factor is not the "title" of the position, but they perform at the highest ability, so come the end of the semester they get a good review. These reviews are placed in their ROTC file and that is the beginning of their career path within the det.
Good luck
