He's guaranteed a seat at Aviation Preflight Indoc (API) so long as he upholds his end of the bargain -- passing OCS, graduating, commissioning, passing TBS, passing a flight physical. If he can do all that, then he is provided the chance to prove he has what it takes, so to speak, to stay in the program. If he passes API, he goes to Primary. From there he'll select his pipeline based on 1) needs of the Marine Corps 2) performance in primary 3) preference. If he succeeds all the way and makes it through Advanced, he'll wing as a Naval Aviator in whatever airframe he selects out of there (based on same criteria as above) I prefer not to think of it as a 'guarantee' as that leads to a lot kids thinking they are somehow entitled to it. Think of it as a contract - you uphold your end, the Corps upholds their's.