Field help (PURPRITYPB) 

This flag is used to determine whether the amounts of the Customer BP Invoices are expressed ex-tax or tax inclusive.
The type of amount of a document is initialized 'Ex-tax' or 'Tax incl.' depending on the type defined at the supplier level.

If the supplier is connected with a supplier category, this information is initialized by the amount type defined at the level of the field Price type/Amount type of the category.