Heading Numbers Add-On requires access to read and change your Google Document in order to do its magic. No information is transferred to any other server or kept by me in any way or form. Everything stays within your own document.
To distinguish between paid (premium) users, and other users, the add-on requires access to your google account information. This information is not transferred, given, sold or in any way kept by me or the app. Paid users email addresses is stored on Google cloud to give premium features.