BAG_BANK and BAG_SUBSCRIBER_BANK are two different bags. This isn't just a number or funny quirk, it has actual consequences in how the underlying game behaves.They are completely separate, and just display as combined when viewed in the game UI.
So if you ask for the first empty slot in the BAG_BANK and it's full, you'll get nil even if sub bank has space. Or if you stack items in the bank, it won't stack an item in the bag_bank with one in the bag_subscriber_bank even if they are stackable (See
https://www.esoui.com/downloads/info...nkStacker.html for a fix
)
If you go through your bank, for items that appear in both lists, you'll probably see separate stacks of both items - and doing Y to stack the bank won't change that.
House storage could actually theoretically work similarly - all items in storage are available for withdrawal when you're inside a house. An addon could be made that shows you all 360 slots whenever you interact with any house storage item.
I haven't tested it, but it might even be possible to put in items above the bank 'limit' if your ESO+ lapses and you had most of your items in the sub bank.