PB00 and PBXX are not fields, but PO condition types.
The condition type indicates, for example, whether, during pricing, the system applies a price, a discount, a surcharge, or other pricing elements, such as freight costs and sales taxes. For each of these pricing elements, there is a condition type defined in the system.
The condition type is used for different functions. In pricing, for example, the condition type lets you differentiate between different kinds of discount; in output determination, between different output types such as order confirmation or delivery note; in batch determination, between different strategy types.
Both PB00 and PBXX are standard “Gross Price” condition types. PBXX is mostly used for manual pricing conditions.
Hope this is useful.