Apply Discount to Product in Multiple Categories

Hi,

There is no such option. The prices and discounts for a product are loaded the same way regardless from where you access the product.
So when you restrict a discount to a category, the discount will apply to the product as long as it is attached to that category.

If you want to change that behavior, that would require some custom development. It would require changing the getPrices and getListingPrices functions of the file administrator/components/com_hikashop/classes/currency.php in order to load the discounts with custom restrictions, and it would also require modifying the product listing in order to pass the category in the URL when access the product page, or when clicking on the add to cart button. It would also require a custom field of the table "item" to be able to store the category from where the product was accessed in the cart and in the order so that you can store the data in that field to be able to use it in the getPrices and getListingPrices functions.
So it requires quite a bit of knowledge to be able to do that.