If there's a hard and fast algorithm that folks understand, I'm curious about this situation: I got tickets with miles for me and my teen on a CRJ-700 (technically a Sky West flight, if that matters). Only a few "free" seats were shown as available, no two of them together. Almost all of the Main Cabin Extra seats were open, but I didn't feel like spending $200 for a short "free" flight. So I didn't select seats at all, and am throwing us on the mercy of the claim that "we will try to seat you together".

Does the system hold back seats, such that even though it didn't show any two adjacent seats, there actually might be some for the computer to assign? If the only two adjacent seats are in Main Cabin Extra, will it put us there? Or will we just get the last two random seats at the back of the plane?