First off, I think that both you and anyone who might be inclined to answer your questions in future would find it easier if you'd just condense all questions you have into a single post rather than taking up space with three. The site owner is concerned with bandwidth, and I'm sure he'd appreciate it if you could conserve some space.
In answer to your question, BOSN is a boatswain, which is a navy trade specializing in seamanship. LMN is a lineman; these guys specialize in constructing and maintaining line based communications systems, kind of like the guys who work for the civilian phone company. Med A is medical assistant, this is what we call our medics, and if I'm not mistaken, Med Techs are preventative medicine specialists, and if you're just joining, then don't worry about that trade, because you have to be a Med A before you can remuster to Med Tech anyway. Finally, MSE Op is Mobile Support Equipment Operator. They drive trucks and other big vehicles.