It is a wrapper around Kiwi Appliance Builder.
A project KIWI, originally supposed to be named after New Zealand citizen, ended up in the community to be just a sour berry fruit. Hence the name: berry(fruit) mill.
Berrymill is an appliance generator of root filesystems for embedded devices. It integrates Kiwi image builder to the Ubuntu/Debian distributions, allowing building images locally.
Documentation is available online at berrymill.readthedocs.io. It relies on the Kiwi documentation and only describes the difference as well as own plugins.
That can also KIWI do. Except if your image build also should equally run on OBS and locally without image description changes.
You have one image that you do not change that often (or at all), but you have little small deviations: add a package here, remove there, change size, filesystem type etc. You don't want to have carbon copies all around the place, but you want to have derived images.
Unlike plain KIWI, which keeps configuration in a straight-forward fashion, Berrymill is also handling the Configuration Management in a framework style.
Berrymill is in early development and some features might be missing or not working properly. If that happens, please open a bug report by adding an issue here.