Apk - Apk file is the main app package file. An app can have single apk file or multiple split apk files. Max size is 100-150 MB.
Obb - If an app has download size larger than 100-150 MB, it contains additional files known as obb files. Usually games have obb files.
App data - App data consists of files generated and downloaded by an app after being downloaded from Google Play Store.
Ex. additional files downloaded by a game.
App data can also contain cache files, user settings and game save files.
Protected data - Protected data cannot be accessed without root access. It usually contains user data such as user settings, game save files, login information etc.
Note: What gets stored as app data and as protected data is entirely up to the app or the game.
An app, as mentioned here, refers to apk and obb files collectively.
An app's data, as mentioned here, refers to app data excluding protected data.
Note: In the app details window, app size is mentioned on the left of '+' symbol, while data size is mentioned on the right.
Backup + Restore can backup an app and its data.
Backup + Restore does not backup:
1. Media or files downloaded by a browser or any app (other than app data)
2. Images or videos captured by camera
3. Projects created by photo and video editors
4. Backups created by a chat or scanner app not stored as app data.
5. Protected data which might also contain user data.
Note: Due to limitations in Android 11, obb and data cannot be backed up on devices running Android 11.
Restoring data is mostly recommended for games which store large game assets there.
Some apps do not recognize data unless protected data corresponding to it is also present.
Should this happen, you must uninstall and restore the app again without data.
Practice extra caution while replacing data as replaced data cannot be recovered.
Note: In any case, restore/replace data at your own risk. We merely provide the option to backup and restore it.
Xiaomi's MIUI being a custom skin, has additional optimizations on top of standard Android's own. You have to enable developer options and turn off MIUI optimizations for being able to install apps.
Step by step guide:
1. Go to Phone settings > About phone > MIUI version. Tap on MIUI version 7-8 times until it says, "Developer options enabled".
2. Go to Phone settings > Additional settings > Developer options > Turn off MIUI optimizations.
3. Now you can install apps from 'Backup + Restore'.