It is pretty simple. The Marines are going to offer you a job field, NOT a specific job. When I signed up, I wanted to drive a tank, they explained that I couldn't just drive a tank but that it was a possibility inside a field. You can sign up to be an engineer because you want to be a combat engineer but you might get stuck in bulk fuel, you can sign up to be a Tanker but you might end up driving missles around on a flat bed. Pick a field where all jobs interest you and don't ever believe they can guarentee you a specific job, only the field. the first two numbers of your MOS will be the field, 03 for grunts, 13 for engineers then you will have two other numbers 0311 = Rifleman 03 = infantry, 11 = rifleman. those first two numbers are all they can give you, the second two you will find out leaving whatever combat school you go to after boot. Good luck!