principal noms

A principal nomination "virtually guarantees an appointment". If you are 3q'ued (medically, physically and scholastically) and you get a principal nom, you are in.
What does it mean to get a principal nom and how do you know what you got

From page 4:
Nominees may be submitted in three categories: without ranking, with a
principal candidate and nine ranked alternates, or with a principal candidate and nine
unranked alternates.7 When the Member specifies a principal candidate, that
individual will be appointed to a DOD academy as long as he or she meets all other
admission criteria. If the principal candidate is disqualified, the service academies
will appoint the first fully qualified, ranked alternate, if specified by the Member. In
circumstances where Members do not specify a principal candidate or ranked
alternates, one individual from among the Member’s nominees who is found to be
fully qualified will be appointed by the academies to serve as cadets.

Sometimes it is mentioned in the MOC's spplication materials what type of nomination selection process he/she uses. Most MOCs submit an unranked slate. If your letter from your MOC telling you of your nomination does not specify whether you received a principal, call and ask.
An admissions officer at USMA told me where I stood, third in line for my nomination :)
Here's how it works:

MOCs may nominate up to 10 students for each vacancy they have each year at each SA (other than USCGA, which doesn't require noms). Most MOCs have one vacancy at each SA most years and occasionally two. It is extremely rare for an MOC to have zero or three but it could happen.

The MOC can nominate candidates using one of three methods. The method used is ENTIRELY up to the MOC. The methods are as follows:

(1) Principal nom and remander ranked. In this situation, the MOC selects one candidate to be his/her "principal" nomination. This means that, if that candidate is fully qualified (academically, physically, medically), the SA MUST give that candidate an appointment. If that person is not qualified, the SA MUST go to the candidate ranked second and, if that person is fully qualified, offer an appointment to that candidate. And so on down the list until they find someone who is fully qualified.

(2) Principal nom and remainder not ranked. This works the same as above as far as the principal nom in that if he/she is fully qualified, he/she must be offered an appointment. However, if that first person is not qualified, the SA can select any one of the remaining 9 (or fewer) individuals on the list. This lets the SA pick which of the remaining candidates it deems most qualified.

(3) Unranked. Here, the MOC just provides a list of 10 (or fewer) candidates and lets the SA pick the one who is most qualified.

Your MOC may or may not proactively tell you which method he/she uses but I believe that most will tell you if you ask. As the above poster said, you may also be able to find out from the SA.

Those candidates who are not offered the MOC appointment through this process go into the national pool -- and a large number of appointments come from that group.

Hope this helps.

One other thing. MOCs can change methods from year to year (i.e., if one year there is a candidate they REALLY want to get an appointment, they can go the "principal" route) and can use different methods for different SAs.

It's something over which you have zero control. You can only do your best to present the best possible record and then see what happens.