This field is enabled only if the Post Taxes on Invoices option is checked in AR Company Parameters.
For Invoices and Adjustments, taxes are calculated automatically, and default an amount based on the tax code specified for the line and the tax basis. If the item’s amount is negative (–), the default calculation in this field will also be negative.You may override default calculations if necessary.
For Credit Memos and Write-Offs, taxes are calculated based on the actual credit or write-off amount, not the original amount. For write-offs, credit tax cannot exceed tax due.