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.
In this field, you enter the tax rule associated with the BP: this tax rule will be suggested by default in all the transactions used by this business partner.
In a multi-company and multi-legislation context, you can set specific information by company, such as: the tax rule, the payment condition, and the default BP discount code.
This setup is performed on the BP/Company tab of the BP record.