Upon arrival in SEL, just as you get outside the jetway there will be Korean personnel asking for transit passengers. You will be a transit passenger. They will ask for you passport and tickets. After having your carry on baggage x-rayed, you will be put on a bus to terminal 1. In terminal 1, you will be escorted to a transit desk where they will issue you a boarding pass for your Thai flight. You will not have to clear immigration or customs. You will not have to pay a departure tax. Your baggage will be checked thru. In terminal 1, there is a upstairs transit lounge with a resturant, snack bar, smoking area, game room and a United First Class Transit Lounge. If you are traveling First Class on Thai you may be able to get a pass to the UA Lounge.