Field help (POHVACBPR) 

This information is mandatory and is initialized to the tax rule associated to the supplier entered in the order header. It is used to define the tax rule to be applied to the order and is used to determine the tax codes to be used for the product lines.
SEEINFO This information is controlled by the BP tax rulemiscellaneous table.

Only a tax rule consistent with the legislation and group of the company associated with the site can be entered.
SEEREFERTTOThe general principles linked to the multi-legislation setup are
detailed Here.

From this field, you can:

If this information is modified during the creation of the order or on its duplication and if the order lines exist, a question will be posed to the user about whether to re-calculate the prices and discounts for the order lines, namely when the price lists are expressed including taxes. When the order is created, this information is no longer accessible.

Inter-company specificities: In the case of an inter-company or inter-site order, the tax rule entered in the generated sales order is initialized with the tax rule of the customer associated with the purchase site.