Sonic Infinity is a downloadable level for Sonic Lost World.
It makes use of the Infinity Base by changing the zone by Play Set Pieces. For the Starter Pack Play Set Piece, it gives you the choice of the Play Set you want to go to. With out a Play Set Piece, it takes place in a level based on the Toy Box with the standard themes and Toy Box objects.
If you play Disney Infinity after playing the Sonic Infinity level, you get Sonic themes and objects based off of the original 1991 game's Green Hill Zone [and partially based on Sonic Generations' version of it]. You also get a Sonic based Adventure.