You can learn more about the different aspects of Offline & Sync in the corresponding section.
Caching on the mobile device
By default, AppSheet does not cache spreadsheet data on the server. However, for read-only apps, server-side data caching significantly improves the speed perceived by app users during a Sync. Explained in simple terms, the mobile app can get the data directly from AppSheet's servers without having to wait for the data to be fetched from the backend cloud storage platform (Google Drive, Dropbox, etc).
By default, AppSheet does cache spreadsheet data on the mobile client in order to allow continued app usage despite transient loss of network connectivity (for example-- getting into an elevator).
However, full and seamless offline behavior needs to be explicitly enabled in the app definition. By default, AppSheet does not cache images and documents that are referenced by the spreadsheet data for offline access. You can explicitly require AppSheet to cache images and documents by setting the appropriate option in your app definition. After you do so, image downloads will happen asynchronously during initial app load. Subsequently, the entire app and its image/document content will be available offline.
By default, your device needs to be online to launch an AppSheet app from the home screen icon. Of course, it can then function despite transient loss of connectivity. If you need the app to launch when offline, this is an option you can explicitly set.
These features are set from the Editor>Behavior>Offline & Sync tab.
Your data is never stored on AppSheet's servers.
The AppSheet web service is an intermediary between the mobile app and the backend spreadsheet. Importantly, it does NOT have a persistent copy of the spreadsheet so there is no danger of your data being compromised via the AppSheet web service. There are two caveats to this statement:
You can optionally ask AppSheet to cache spreadsheet data in our service in order to improve performance. If so, this data is cached in-memory in our server for up to five minutes at a time.
AppSheet caches resized copies of images used by the apps. Image resizing is important to conserve network bandwidth to mobile devices.