XX is a boss attack-type of level where the chosen character must again fight all the bosses that were previously fought in the game. They are easier to defeat this time around, as they require fewer hits. The final boss for this stage is a giant mech that is controlled by Dr. Eggman in person. Once the giant mech boss is destroyed, the main quest for the character chosen is completed (except Sonic who has another stage to pursue after Eggman as Super Sonic but unlike Sonic Advance, the extra zone doesn't follow directly after the final zone and so the player is required to go back into Sonic's story mode and select the extra zone from there after the credits roll). He/she will descend through the air while the space station behind them crash lands at some place in the world, creating an explosion. Netherless, the player's character lands at a meadow (in Cream's case, she is reunited with her mother), strikes a pose at the player and then the credits roll.
The stage's name (XX) is a reference to the X-Zone from Sonic Advance which has the same stage concept.
If the player clears this final zone, the music of the zone selection map would change to a remix version.