Field help (SOHVACBPR) 

A tax rule represents the tax territoriality principle, in other words, the calculation rules to be applied to determine the tax amount.

As a general rule, by crossing a tax rule linked to the BP with a Tax level, you can determine the tax code to be applied to the document line and so, to the entry line.

The tax rule is based on the tax rule of (in order of priority):

Once the order is created, this information is no longer modifiable. If you modify the tax rule after entering order lines but before creating the order, then taxes are re-calculated on all entered lines.

Only one tax rule with a legislation and group that are consistent with those of the document can be entered.
SEEREFERTTOSee the documentation on general principles linked to the mutli-legislation setup.

The parameter CTLTAX - Tax codes control (VEN chapter, VAT group) is used to control that the BP tax rule: