You can never select your seat in advance on Southwest. You are issued a boarding pass when you check in. You are assigned to group A, B, or C based upon the time you actually checked in for your flight. Passengers in group A get to board the plane first and can choose any seat. Then group B board, and finally group C.
Effective November 1, they were suppose to change the boarding procedures so that you are assigned a letter and number to indicate your boarding group. Passengers holding boarding passes with Group A1 through A5 board first, then A5-A10, etc. It's designed to make the waiting process easier so people aren't lining up an hour in advance within their group.