An IP1 car runs in CW1. It can also be run in GTO if that is the second entry for the event. If the IP1 car is GT legal with all the required safety items needed for a GT car then it could be entered in just GTO. Somebody can correct me if I am wrong.

If I'm not mistaken it is usually left up to the organizer to determine what cars/classes should run in what run group as they will be the ones to determine if there are an appropriate number of total cars in the run groups to provide good and safe racing. It certainly is a difficult juggling act keeping in mind the possible conflicts what with some cars being entered in two different classes and two different run groups and the potential for disaster with super slow cars and super fast cars on the track at the same time. I wouldn't want that job, but typically the race announcement has the class/run group breakdown listed at the bottom.