Principal noms are not the norm, it usually is the "competitive" nom. Only about 30% use principal.
The MOC is not required to use the same method yr after yr. In other words, if they used principal last yr it doesn't mean they cannot use competitive this yr.
As stated if competitive method is the way the MOC submits their slate, the highest WCS wins the nom. If they use principal and that candidate is 3 Q, the SA will charge them to the MOC.
There is also principal with alternate, and principal with competitive. With alternate it means that if the principal turns it down, it goes down the list to the alternates (ranked) if with competitive, it means if principal turns it down, they are allowing the SA to select using the WCS method.
The MOC will tell you exactly what method they are using when submitting the slate.
Also understand that if you put multiple SA's on your list for a nom., they may give you a nom that was your number 3 on the list too. Thus, becareful to make sure you want to attend any and all of the SAs. Yr after yr on this forum kids will say I got a nom to this SA, but I don't want to go there, is there anything I can do to switch it? You must understand the MOC's are not mind readers, if you put that SA on your list it means you want to go there. It will not be held against you if you only ask for 1 or 2 SA's. DS put only 1 and got all 3 noms to that one SA.
Good luck