What is On-device encryption... and does that mean only Pro plan users have secure data?

What is On-device encryption… and does that mean only Pro plan users have secure data?
Does the following statement apply to Premium users or only Pro.
When users sync their app, changes they make are sent to the AppSheet web service over an encrypted protocol (HTTPS). AppSheet then applies the changes to the backend spreadsheet (on Google Drive, Dropbox, etc). The latest version of the spreadsheet is read (from Google Drive, Dropbox, etc) and sent back to the mobile app.