The first to board will be the pre-boarders. I believe they just line up in any order. I don't think pre-boarding uses any sort of numbering scheme.
Boarding positions A1-A15 are reserved for Business Select. By reserving that section, they can sell someone a Business Select fare up to the point where boarding is due to start and still give them a low boarding position. Even if A1-A15 was full (and I've never heard of that happening) they would have any Business Select customers board before A16. Any open positions in that reserved section are available for purchase 45 minutes prior to departure for $40 each. Even with that option, I've never seen more than 5 or 6 people in A1-A15 so most of that section is usually empty.
The order of boarding assignments for A16 and beyond starts with A-List Preferred, then A-List, then Anytime fare customers with EBCI and finally Wanna Get Away with EBCI, all in order by date/time of ticket purchase for A-Listers, date/time of EBCI purchase for Anytime and WGA. A-Listers who get a "B" boarding position are allowed to board at the end of the A line, ahead of family boarding which occurs between the A group and the B group. After this come those who did not purchase EBCI but checked in starting at 24 hours prior to departure.
It is quite possible to pay for EBCI and end up in the B group. So even though you paid for EBCI, if you're in the B group those using family boarding will board ahead of you.
It's also possible to snag a low boarding position by checking in a few hours before departure. For example, an A-Lister with A18 cancels his/her flight a few hours before departure. The next person to check in for the flight gets A18 rather than a position at the end of the B's or somewhere in the C group. In other words, when a cancellation occurs, they don't leave a "hole" in the boarding queue, they give that position to the next person who checks in for that flight.