By default, all users of an app work with the latest version of the app. As the app creator, you might want to make changes to the app definition. These changes are immediately visible to the app users after a sync.
In some situations, you might prefer to work with a new app version for a while without affecting the existing users. AppSheet provides a mechanism to do so. You can designate a Stable version of the app on the Manage > Versions pane of the app editor.
Each user of the app can be assigned to either the Latest version or the Stable version of the app. This allows you to leave most users on the Stable version, build a new version, and gradually add a few test users to evaluate it. When it is ready for broader use, you can mark this as the new Stable version and all the users will now work with this version of the app. See Manage app permissions for individual users.
The Stable version feature is only available with certain subscription plans.