I have a request for an battle pouch for each actor where you have to equip items on. Each actor can only bring in a certain amount of items. You can put note tags on actors, classes, armor, or items to determine the amount limit an an actor can equip  in their battle pouch or side deck. On the Driftwood Discord, I was talking to you about a card battle system to as a game type for your grid battle system. These items would function like one time use cards called Treasure Cards. Another inventory would be the Extra Deck. It would just used to hold Item Cards, which are cards that are automatically added to the deck as along as your actor has a certain piece of gear equipped.  The side deck and extra deck would be tied to the Main Deck by a custom piece of equipment called a "deck". The deck can have note tags that allows for setting deck size, card capacity, card costs and deck limit, side deck limit, and item limit. Just trying to get ideas out there.

I think it would be an interesting implementation of multiple inventories. indeed, a "deck" of cards can be represented as an inventory, and therefore you could have inventories that represent

• your hand
• the main deck
• the side deck
• the extra deck
• the graveyard

If we consider each card to be an item, then various card/deck operations such as

• drawing
• discarding
• peeking at the top N cards
• shuffling

Can all be treated as various inventory operations.

Having a physical "inventory" item that is used to determine what your current deck is would also be an excellent way to handle things.

If you could event the battle logic to move cards from one inventory to another, then you'd have a lot of control over the rules of your card game.

Exactly, better concept than just having a menu to add and remove cards from  the deck.