Hi SP and Welcome,
 
Collections has 2 purposes:
1.  A place for cards that are not Announcement, Business, Birthday, Holiday, Invitation, or Occasion cards.
2.  GCU allows all cards to go in up to 2 categories, and Collections has some catch-all subjects, like humor, where funny cards from the other 6 categories can find a second home for instance.
 
There are also many categories in Collections that are unique, such as blank cards and note cards, which are not allowed in the other 6 major categories.
 
The important thing is the card does need to clearly match the category you put it in or it will be sent back to you to resubmit.  That's no fun!   If you see many cards already in that category that match the one you want to put there, then you should be OK.
 
Does that help?
 
TOM