The stock recovery on Android device can delete all user and cache content ( for factory reset purposes ), allow external tools to run functions on Andorid device and perform system updates. The stock recovery is a limited system. A custom recovery is a third-party recovery environment. Compared to stock recovery, the custom recovery has more addtional features. It is modified to allow update packages that have not been digitally signed by official sources. Custom recoveries has the ability to create and restore device backups.