Price List can be automatically called from many places. The following is the order for this.
a) From Transaction Option - When you start the entry then it will set the price list defined in the option. You may set it to [None] is you do not want to use this.
b) From Customer or Supplier - If you select a customer it set the price list defined in the customer. Before setting this it will ask to change the prices of the products already scanned. If you select yes then the price of all the items will change and the price list will be set for next items.
c) Manually by User - If you want you can select from the available price list by using "select price list" button. If you already added some products then it will ask you about changing the prices. You can choose Yes or no based on your requirements.