Note to self: check sources before posting.
upon further reading this is how I understand it to work.
1) if multiclassing into a service you can't get your first level as a marine, navy, army, scout -- until you've served your first term. ie any experience prior to enlistment must be spent on some other class.
2) However you can join a service class with experience in some other class under your belt. you can only level up IN a service class while actually employed in that service. Core classes have different rules: IE merchants can only level up while in a merchant prior history OR while part of a working starship. Academics, rogues etc. can level up whenever they gain sufficient experience (ie a character with one level of army and 5 levels of rogue, might never have actually had a 'rogue career')
3)Once you leave a career in past history you can never return to that -career- (meaning those tables on the prior history. however, unless you were army navy, marines or merchant. you can continue to level up in the class of the career you just left instead of in the class of the career you are serving.
erg.. I'm getting tied up in knots.
did any of this help or did I confuse you further?