The main purpose of a backup is Data Loss Prevention. Therefore it is recommended to perform a backup after a new installation of CoreELEC, and before any update or change to the system.
CoreELECs internal backup tool backs up and restores only config files and not the OS itself, including: usernames, passwords, settings, add-ons, configurations files, personal api keys, pictures, video and audio files, thumbnails, temporary files, etc. But not the operating system itself (like linux and kodi which are changed by CE update procedure)!
Settings
CoreELEC
System
Create System and Kodi Backup
The backup will start
HINT: before removing the USB Flash Drive, reboot CoreELEC from the power menu, allowing the backup file to synchronize to the drive.
Settings
CoreELEC
System
Restore Backup
Yes
to restore the backupCoreELEC will restart
Upon reboot the restoration process, which can take up to 10 minutes, will begin.