When you are selecting products that a coupon applies to, you're essentially stating 'product 1 AND product 2 AND product 3', so if a customer adds all 3 products to their cart, the coupon will apply to all of them.
Say I've got a 50% off coupon code, and I want it to apply to any of the t-shirts on my store. But I only want it to apply once, at one use per customer. Currently, the customer could add all of the t-shirts in my store to their cart, and get 50% off of all of them.
How can I make it so that the customer only gets a 50% discount on one t-shirt (regardless of which t-shirt it is) without changing the coupon to deduct a fixed price?