Item groups can be used to specify which items qualify for a discount provided by a gift, punch card, or club deal.

If you'd like to include different items in a discount (even if it's only adding or removing certain items from the item group), you need to create a new group for the desired items.

From Campaign Center > Gifts > Choose a certain Gift > Terms > Add Condition > Select Item > Saved Groups > click on the Edit pencil icon. You can create a new item group from the Scenario tab: