Basically. If you try for the trades that you want the most, it is decided that you will not be extended an offer, you will be given an opportunity to apply for some other trades. If none of those other trades are appealing to you, you can of course re-apply the following year. Depending on a few things. For example, DEO applicants for the trades in the Naval Operations branch (MARS, CSE, MSE) are sent to the Naval Officer Assessment Board. There, they spend a week (well, 5 days) both being evaluated, and getting to know more about the navy, what we do, and how we do them. At the end, you'll be told one of three things. A) You made the cut; B) You're considered an acceptable candidate, but you didn't make the cut; and C) You have been deemed to not be an acceptable candidate. Should someone fall into the C category, I would advise them to not pin their hopes upon re-applying next year. I mean, I suppose you can try, but I don't think there's anything guaranteeing that you'd even be invited back to the next NOAB.
Oh, and someone can be told they didn't make the cut, and then later be extended an offer. This will typically occur when someone else who had been extended an offer either later declines this offer, or injures themselves preventing them from starting BMOQ.