Field help (TRGPOSTTAX) 

The value of this field refers to the field VATIPT in the Accounts function. If a line type is parameterized as Subjected, Tax account or EEC tax, it is possible to set up a dedicated link for the values Collected Sales, Collected Fixed Assets, Deductible Purchase, Deductible Fixed Assets, Deductible G&S

For Germany/Austria usually only the types “Collected sales” and “Deductible purchases” are used.