When processing promotions, B2C Commerce starts by generating a list of active promotions based on parameters such as customer, source code, and coupon code products. This list isn't just for applying promotions to a basket. It also to present potential promotions to shoppers, such as on a product details page.
Then B2C Commerce identifies applicable promotions based on basket contents and priority rules. Finally, the promotions are applied to the basket.
By default, promotions are sorted by their discount types. If multiple promotions have the same discount type, the discount amount determines the rank order. The highest amount wins.
B2C Commerce can't identify which discount a multi-tier promotion uses, so it pre-evaluates all promotions based on the discount plan without applied discounts. Although this is an approximation approach, it ensures a predictable sorting order. However, sorting by the highest promotion class/type priority rule or discount type doesn't always ensure that a rule or discount tier applies.