The default purchaser type, A, on the site can see all the products in the catalogue. Type B Purchasers only see products that have a price in the price list they are associated with. If this is a short list then when they log in they simply see a page of products that they can purchase. If it is a long list then navigation can be created dynamically using the Product Category and Sub Category which can be configured manually in Shopfront Designer or synchronised with your accounts system.
If the lists of products are short for every customer then it can be obtained from the accounts system every time the customer logs in. If the lists are larger then they will be loaded each night by a scheduled task.