Displays only if allowing discounts on invoices and tax discounts are in use (options in AR Company Parameters).

Specify the tax discount offered for this invoice line. Initially defaults an amount based on the line's tax rate and payment discount amount. For example, if the invoice amount is 100.00, the tax amount is 5.00 (rate of 5%), and the discount amount is 3.00, this field will default a tax discount of 0.15 (5% x 3.00).