If you are using a supplier catalog such as IMC or WorldPac, you use the link on the admin.epc page called - Add Parts Pricing (supplier). This page describes in great detail how the actual pricing forumula works.
Towards the bottom of the page you are provided with a drop down menu of available brands and or product categories. You can mix brands and product categories, for example you can set a pricing model on BOSCH SPARK PLUGS, or ATE BRAKE PADS.