VAT in Invoice Office comes from a tax code, not a number you type on each line. The code carries both the rate and how the sale should be treated, and that pairing is what keeps your VAT return correct rather than merely plausible.
Where tax codes live
Settings → Tax codes is where the codes themselves are managed — the rate each one carries and how it is reported.
Add a code here when you need a rate or a treatment you do not yet have. This is the right move whenever you find yourself wanting to override a number on an individual invoice: the override fixes one document, a tax code fixes the category.
Changing the VAT on a document
Open the document and change the tax code on the line. The line total, the VAT total and the document total all recalculate immediately.
Different lines on one invoice can carry different tax codes, which is exactly what you need when an invoice mixes a standard-rated service with a zero-rated one.
Set the default so you rarely touch it
If you are changing the tax code on every line, a default is wrong somewhere:
- On the product — set the code the product is normally sold at, and every line created from it starts correct.
- On the customer — their settings hold a sales tax code, which handles a customer who is always treated differently, such as a business in another EU country.
Between the two, the code on a new line is usually already right.
Legal text belongs on the code
A reverse-charge or exempt invoice has to carry the right wording as well as the right rate. Each tax code can hold its own legal text, per language, which is placed on the document automatically whenever that code is used — so a cross-border invoice explains itself without anyone pasting in a sentence.