Manager substitutes Tax Invoice as the title of a sales invoice, in place of simply Invoice, when specific built-in tax codes are used for countries where it is known that regulations require that. When most built-in tax codes or custom tax codes are used, that substitution does not occur.
So the questions to you are:
- What country are you in?
- What tax codes are you using?
- If your country requires a sales invoice to be headed as VAT Invoice, can you provide a link to the legal requirement? (The software will not be changed without appropriate documentation of the need.)
Meanwhile, if you personally want to change the title of a form, you can change it to anything you want, whether a legal requirement exists or not. See this Guide: