Belt
The Belt is the scrollable area at the top of the Editor which stores all of the Blueprints in a card.
If you're new to Castle, the best way to learn how to use the belt is to follow the guide How to make your first Castle deck or Castle Academy.

Adding new blueprints to a cardโ
To add new blueprints to the current card, press the [+] button on the left side of the belt. This opens the Library sheet, which lets you choose among Castle's default blueprints, premade kits, your own decks, or decks you've liked elsewhere in the app.
Adding new actors to the sceneโ
After you've added some blueprints to a card, you can put new actors in your scene by dragging them out of the belt downward into card space.
Selecting blueprintsโ
Tap on a blueprint in the belt to highlight all instances of that blueprint in the scene. Press Edit blueprint to open the Inspector for that blueprint.