Price list is a collection of items and their assigned prices. An item can exist in multiple price lists. This gives you the ability to assign different prices to the same item. Price lists let you set currency and duration of the prices. For example, if the price list expires, prices of items within the price list will also expire. You can configure the pricing API to include your default price.
When requesting pricelist of items or evaluating promotions, you should mention the price list ID that you want to use.