And if you have an LOA, mentioning it to your Member of Congress (MOC) contact prior to your interview with their nomination board staffers is not a bad idea (assuming that you haven't had all your interviews yet). More than likely - they will already know of the LOA - but sometimes news is slow, especially if you find out you have an LOA on the DoDMERB site before you actually receive the paper copy in snail mail.
What the LOA says is that the Academy likes what they see in your complete application and wants to give you an appointment. If there are caveats or conditions tied to the LOA (there almost always are), then you have to meet those conditions (Pass DodMERB physical, get a MOC nomination, etc) in order to get the appointment.
It's a "breathe easier" moment, but not a true assurance that you will receive an appointment. Odds are in your favor, however!