Kenny 2x4

What you got to do is go into Challenge Mode and you will notice there are 2 missions for every level for each team.
The common trend is as follows:
Team Sonic - Reach the goal ring in the time limit given
Team Dark - Defeat 100 enemies
Team Rose - Collect 200 rings
Team Chaotix - This will depend on the Story mission for this team for example. In the Seaside Zone level in Story mode the mission was to find and collect 10 Hermit Crabs. In the "Secondary" mission as it were you would have to find all 20 of them.