By default existing VAT rules are applied automatically while adding product with VAT in Invoice line.
To edit VAT settings per Invoice line manually:
NOTES:
• It is allowed to select only those VAT Rules for a product which are specified in product's profile.
• Right after selecting VAT Rule, VAT becomes recalculated based on made selections.
• Destination and Origin affect only VAT rules applying. If VAT product and Calc schema is set, it is calculated correctly no matter of destination and origin.
• It is not allowed to set calculation schema until there is no ‘VAT Product’ selected, because it depends on the selected VAT.