Enlighten me on what a "principal" nomination is. Are you talking about Presidential nomination?
A principal appointment has nothing to do with presidentials. A senator or representative is allowed to make their slate of 10 nomination, 1 of 3 ways.
1. Competitive: Most common. The senator or representative makes their list of 10 names of nominees. They simply give that list of 10 to the academy and say: "Here's 10 names..... PICK 1". In "THEORY", the academy is "SUPPOSE" to pick the most Qualified individual on the list of 10. That individual receives an appointment against that senator or representative's slot. The remaining 9 go into what is commonly called the "National Pool" where OTHER appointments come from.
2. Ranked: This is where the senator or representative gives the academy their list of 10 names, and the names are RANKED in order: 1-10. 1 being the PRINCIPAL nominee. 10 being the LEAST. In this scenario, assuming the PRINCIPAL nominee is qualified according to the academy's standards, the academy has NO CHOICE. They MUST give that individual an appointment and they are charged against that senator or representative's slot. If the PRINCIPAL individual is NOT QUALIFIED, the academy MUST go down the list and take #2 if they are qualified; and so on. Once the appointment is made, the remaining names go to the "National Pool" just like in scenario #1.
3. Hybrid: Basically, it's a combination of the first 2. The senator or representative presents the academy their list of 10 names. The 1st individual is their PRINCIPAL. The remaining are just 9 additional names. The academy MUST give an appointment to the PRINCIPAL if qualified; (As in scenario #2). If the individual isn't qualified for whatever reason, the ACADEMY can choose from the remaining 9. Again; supposedly based on the MOST QUALIFIED. The remaining goes into the "National Pool".
I prefer the RANKED method. If forces the senator or representative into taking their responsibility seriously and being the one who chooses who gets the appointment instead of being able to blame the academy. Plus; even if every senator/representative used the RANKING method, on average, that would only be about HALF of the appointments. (There's only 535 senators and representatives). Average 1 each per year. That still leaves the academy with approximately 500-700 appointees that THEY get to determine who receives the appointment.