Auto Upgrade is a feature of the Serval Mesh (app for Android) which allows upgrades to be distributed via Rhizome.
The Auto Upgrade technical document gives more detail. In brief:
In order to reach the vast majority of devices that are outside Wi-Fi range of The Serval Project (and without Serval Infrastructure access to The Serval Project), a couple of tricks are used to seed Rhizome networks with new APK files: