There are some number of blocks of various sizes of seats scattered around the stadium. When your turn comes, you can only see blocks that exactly match the number of tickets you previously told them you wanted to buy. Everybody goes in AT priority order, but they can only see and select from blocks that match their number of tickets.
I guess theoretically all the 4-seat blocks could sell out by priority 2000 and the 2-seat blocks could last until priority 4000 or something like that, but I assume they used the info they collected about how many people wanted to buy tickets, and at what block size, when allocating the number of blocks of each size.
The only variable I see is what they do with blocks from people who decided to opt out. It would suck if the 2-seat blocks ran out before the 4-seat blocks and somebody lower priority got tickets because they wanted 4 seats, versus busting up the bigger blocks for higher priority people.
JRjr