You can not set discounts for Menu Items or Menu Categories.
You can only set discounts for Products.
For Products, you can use different methods based on the Group Code, Product Tag, Custom Product Tag, or even the Product Name.
Products are not the same as Menu Items. Menu Items are simply “mappings or links” to Products. And a Menu Category is not the same as a Product Group Code. Menus and Menu Categories are for display purposes on the POS Ticket Screen.
But since we don’t want a fixed discount, we need to update the Price to emulate a % Rate, which we do with the following Rule. The formula in the tagPrice parameter is what makes this “magic” happen …