You can show ads from a set of products to people who interacted with another product set.
In order to achieve this, you need to select "Up-sell" option on the top of "Ad sets" step. After selecting which products will be promoted to the users, you need to choose the products that the users interacted with (related Product Set) in order to define the targeted people. By interaction, we mean viewing, adding it to cart or purchasing the product.
If product sets were not yet defined, you can create a product set on the fly by clicking on "+" button.