The "criteria" section of a promotion is the thing we want the customer to do to trigger a discount. When you think about it a discount is actually composed of two things: The thing we want the customer to do; the criteria part, and the thing the customer gets; the discount part.
Think of “criteria” as “The customer must buy this particular product”, “The customer must buy for more than x amount”, “The customer must buy something from this particular store, catalog, or category”, etc.
Criteria can be combined to form even more powerful promotions such as a product bundle or kit depending on which side of the pond you’re on.
If the criteria is set up so that the customers must meet all, they must all be satisfied in order to trigger the discount. Say you want to the customer to buy two products to get a discount. You’ll set up two criteria: One for each of the products in involved. Ucommerce will look for these products in the cart of they are both found will trigger the discount.
Alternatively, if the backend user selects at least one of the criteria to be met, then any of the selected targets will trigger the discount. That will allow the application of the same discount based on a collection of different criteria without having to create the same promotion for each item in the collection.