mainline: The main tree where the core functionality and core features are being developed.
subsystem/feature branch: is a branch within the same repository. In our case, we will use the term branch also when referencing branches not in the same repository, which are a copy of a repository sharing the same history.
upstream: A parent branch the source code is based on. This is the branch you pull from and push to, basically your upstream.
LTS: Long Term Support