The fields

The template governs the layout, not the content

That distinction saves misunderstandings: the document comes out tidy, but what is in it is on you.

The serial number

You number it and you keep it from repeating. The template does not remember what you issued last month.

Rates and amounts

The rate depends on what you sell and to whom. It is written deliberately, not picked from a list that would guess on your behalf.

The dates

The issue date and the due date are two different dates, and separating them before the document reaches the client is worth the second it takes.

Both parties' details

Full name, address and registration number for both sides. The shorthand that is obvious to you may not satisfy the recipient's accounts.