Field help (SALPRITYP2) 

This flag is initialized based on the business partner. It determines if the amounts on the invoice lines are including-tax or excluding-tax amounts.

It is only possible to choose between Include tax or Exclude tax when creating the invoice.

If the DEPMGTMOD - Discount management mode parameter (TC chapter TC, group INV) is set to Rebate on VAT, the amount type is always Exclude tax, regarldess of the BP setup or value of the PURPRITYP - Amount type parameter (TRS chapter, INV group). You cannot select Include tax in that case.