The default price list is used on new deals for any company that doesn't have a price list added, or whose price list has been deactivated. A company with an active price list gets that price list instead. A deal that has been saved keeps its price list. You can change the price list on each deal in the Price list field.
To make a price list the default, turn on Default list when you create or edit it. You always need one default price list, and it can't be deactivated. See Price lists overview for how to create and edit price lists.
Price lists are part of the CPQ and Advanced Order Logic add-ons.