The Build Appliance is a project under The Yocto Project umbrella. The purpose of the Build Appliance is to use Yocto to build a virtual machine image, inside of which (i.e. when run) one can perform Yocto builds.
Another tangential purpose of the Build Appliance is for testing Pseudo. The Yocto Project uses up-to-date packages for every component on master. An image built with master will include the latest glibc, etc. Using this Yocto-built image in which to build Yocto verifies that Yocto (and therefore pseudo) works with all the latest host packages.
The following document was created when the Build Appliance was initial conceived. It is a planning/design document for the Build Appliance's initial inception: Build Appliance Design.
In time the Build Appliance lost its maintainer and stopped working. In 2023 an effort was made to revive it. Updated notes about the Build Appliance from this time can be found at: Build Appliance 2023.