Field help (VACBPRSOH) 

The tax rule is used to define the various taxes that can be applied to the order. Namely by cross reference with the tax levels of the products.

The tax rule is initialised with the customer tax regime. Once the order is created, this information is no longer modifiable. If on the other hand the tax regime is modified after the entry of the order lines but before creating the order, the taxes will be recalculated for all the lines already entered.