These cards work differently . 

The item cards count toward your card hand limit. They add something extra to an attack. You can only play one of these per attack (normally). 

The supporter cards help your game and count toward your card hand limit. They aren't cards you play but while they are in your hand they help you.

Finally the energy cards give you a hint about something that will happen on the trip. This is important information and these cards should be discarded to stop the secretes from getting out.