The Rose, Sugar, and Orange Bowls all have contractual ties with conferences for both teams to appear. When they are not playoff bowls, they will take a team from those conferences. The Rose Bowl this year, for example, will have a b1g team vs PAC team, no matter what.

The Cotton, Peach and Fiesta have no such contracts.

So in 2016
Playoff bowls are:
Peach
Fiesta
Then NY6 bowls are:
Rose - B1G vs PAC
Sugar - SEC vs B12
Orange - ACC vs B1G/SEC/ND
Cotton - At-large vs G5

in 2017:
Playoff bowls are:
Rose
Sugar
Then NY6 bowls are:
Orange - ACC vs B1G/SEC/ND
Cotton - At-large vs G5 (****the g5 doesn't have to go to cotton in 2017, just a place occupier in my example here)
Peach - At-large vs At-large
Fiesta - At-large vs At-large

Note that some of these at-large bids can disappear if a P5 conf champ doesn't get to the playoff or Orange Bowl, they will play in Cotton/Peach/Fiesta and eat up an at-large bid. If the B12 doesn't get in the playoff in 2017, their champ will take an at-large.