Permissions used by Sintel Forms

Sintel Forms uses delegated permissions to enable users to create forms and edit them. Additionally, if you enable the External Submissions feature then application permissions are also used as this is required to enable external users (those with no access to your tenant) to submit forms.

When you install Sintel Forms and authorize it, the process grants it access to read the contents of SharePoint lists and libraries and read user profile data. All of these permissions are required for the application to work correctly.

More info can be found in the following Microsoft article Add an app to a site.