"In a ""tax-on-tax"" situation (described above), the tax_on_tax_id is the tx_id of the tax that should be included in the tax calculation for the current tx_id. For example, in Canada the TVQ tax includes not only the line items taxed, but also the GST tax for these line items. The proper response to this tax_on_tax_id field would be the tx_id for the GST tax. Although we are currently not aware of any such situation, it may be the case in a jurisdiction that there is a tax-on-tax-on-tax situation. It is anticipated that this field would hierarchically grow as necessary. Using the example in the previous paragraph, say that tax3 includes TVQ and GST taxes. Since the TVQ tax already shows the hierarchical relationship with GST in the tax_on_tax_id field, tax 3 would list the tx_id for TVQ in this tax_on_tax_id field. "