The AFL block seats. They allocate bays/aisles for opposition fans at Adelaide Oval. I bet that the ground level seats of eastern stand will go crows members aisle, cats members aisle, crows aisle, cats aisle etc so there will be be 5 aisles of crows members and 5 of cats members between aisles 131 and 140. That will be repeated across the ground and the AFL will guestimate how many aisles in total to leave for cats members. They might keep that open until Tuesday or Wednesday night for Geelong members so that bay/aisle might not be sold out but allocation is exhausted for crows fans.
The AFL keep aside tickets for corporates, the 2 clubs requirements and other requirements and they block those seats from the public. That means those seats are exhausted by their allocation but they haven't necessarily been sold to people. For example every AFL Life Member has the right to 2 tickets and seats are allocated. Now for an AO game I don't know if they initially allocate seats for every AFL Life Member alive or they estimate how many would travel over. The AFL Life Members have to let the AFL know by a certain time if they will use their allocation. At some point the AFL will put these unused tickets back into the pool. Dont believe it's a sell out until it's officially confirmed only a few hours before the game. They will keep releasing those unused allocations back into the pool until that time.