Southwest uses open seating.
Uhm....Southwest doesn't allocate seats. They allocate boarding groups. They have done some testing of allocated seating in selected markets, but no announcement has been made that it will be adopted systemwide.
Boarding Procedure
Each Customer will be issued a boarding pass grouped by A, B, or C (in that order) based on when the Customer checked in online at southwest.com, at the Skycap Podium, Ticket Counter, Departure Gate, or E-Ticket Check-In kiosk (where available).
Prior to general boarding, Customers with disabilities, unaccompanied children between the ages of five and 11, and adults traveling with a child under five years of age will preboard. Customers who choose to preboard cannot sit in an emergency exit seat.
Because Southwest Airlines maintains an open-seating policy, general-boarding Customers may sit in any open or unclaimed seat. Customers holding boarding pass "A" will begin general boarding, followed by Customers with boarding pass "B," and then "C." General-boarding Customers who choose an emergency exit seat must meet all requirements set forth by the Federal Aviation Administration and Southwest Airlines.