Great question — answering as the UphillTech team:
The Southwest website has a tutorial on how their boarding process works. There are no assigned seats, but when you check in online you are assigned a boarding group and number. Groups are boarded in alphabetical order (so "A" is the first to board), and the number (like "A-14") indicates your spot in line within that group (here, the 14th person). There will be signs that tell you where to stand based on your number. (Online check-in starts 24 hours in advance for some reservations; for others, you have to check in at the airport. The earlier you check in, the better your number is. ) When you board, you just pick whatever seat on the plane you want that doesn't already have someone...
Full answer by Laura: https://travel.stackexchange.com/a/2309 (CC BY-SA 3.0).
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