first, each bowl game is assigned 2 different conferences that will have a representative in the bowl game. For example, the Rose Bowl is Big 10 #1 vs PAC-12 #1. That means that the first place team in the big 10 will play the first place team in the PAC-12. However, if the one of those teams is taken by the BCS Championship then an at-large may be selected for the rose bowl. For example, last year TCU got to the Rose Bowl becuase the 1st place team in the Pac-12 was in the BCS National Championship Game(Oregon). Wisconsin won the tiebreaker for the Big 10 so they got there to play TCU.