On Device Data means your core Unfair information is saved on your iPhone instead of a required cloud profile.
What this includes
Your profile setup, stack state, reminders, dose logs, and journal history are kept locally on your device.
Why this matters
Local storage supports privacy by default and keeps key app flows available even without internet.
Device change expectations
When moving to a new phone, your app data depends on your Apple backup and restore process.
If you skip restore, prior local history may not carry over.
For a full privacy overview, read Local Private Storage.