As of November 11, 2012, the PSUPedia and its affiliated wikis have made the transition to read-only mode. While edits can no longer be made, we hope you will continue to enjoy the data collected by the community for years to come!
From The PSUPedia
Co-op missions are missions which are very similar in style to free missions, however they contain some elements of party missions. Even though co-op missions can be initiated by a single player, they require more than one player to complete with a grade of S due to field elements such as multi-person switches which open doors leading to additional enemies.