This switch gives you the ability to add a line item that consists entirely of VAT. This is used when the exclusive value is zero and the value comprised solely of VAT.
There is no unit price on these Pure VAT lines, only a VAT amount.
An importer will use this function when invoicing their customers for the VAT charged by Customs and Excise.
If you receive invoices from a Clearing Agent, then the VAT on the goods being cleared can be entered using this functionality.