The "Default Price B" option can be setup when an alternative markup over Price A applies to the customer if buying Open Market inventory. This can be set as a Percentage or Dollar amount. The system will take into account the Default Price B setting within the customer account when providing pricing for Open Market inventory. This markup also applies anywhere on the system where price is suggested from a price list.
The "Default Price B" also needs to be active if a Price List is being assigned to the customer in order for the pricing within the price list to apply. For more details please review Price lists.