Currently, the only way of controlling which products that are included in a discount of type Discounted product price is either by selecting single products, or by using a campaign price list. In some cases this is not enough, and being able to also limit to a set of products included in a product list would be useful.
Use cases:
Create a discount for all products in a campaign price list that are in stock (by using a smart list)
Create a "sub-campaign" for all campaign products of one brand, or products in one category