Field help (EXCTAXRUL) 

Field used to specify whether the element must be included in the calculation basis of the levy. The allocation of an invoicing element to the basis used for the levy calculation is only performed when the basic rule of the invoicing element is "Product price" or "Distribution".

SEEINFO The invoicing elements generated from the discounts or charges of document lines (relative to the Price list structure setup) are not included in the calculation basis of the levy.