In the config file there are system defaults, which are used when building the image and for automated builds.
* **GITHUB_USERNAME** - Used if you want to automatically log in to the box without typing a password. This is used at build time.
Then theres other raspbian stuff, that you may find in the [Raspbian documentation](https://github.com/RPi-Distro/pi-gen/blob/master/README.md) which will still work.