Combat Arms, Combat Support and Combat Service Support functions can't be contracted. Those are largely comprised of 'pointy-end' combat-specific or related occupations with battlefield tasks that have no real civilian equivalents. 
Well, I suppose you could contract them but then they're called mercenaries, though the newish PC, polite, technical term is Private Military Corporation (PMC). Dubious legality and not even remotely in consideration (for good reason).