The kernel sources are divided into subsystems according to their functionality.
Maintainers
A maintainer should be assigned to each subsystem.
The maintainers are responsible for accepting a patch for the corresponding subsystem.
When the merge window opens, the maintainers send selected patches to Linus Torvalds to merge them into the Linux kernel git repository.
Mentioning subsystems in patches
Subsystems are often (and as I believe even required) to be mentioned in the subject line of a patch.
The following git command shows the subject lines for the patches that modified a file in the kernel sources and demonstrates that they often refer to the same subsystem (the word left to the first colon):
$ git log --pretty=oneline --abbrev-commit Documentation/conf.py
34d9f62e0456 Documentation: conf.py: Add __force to c_id_attributes
eaae75754d81 docs: turn off "smart quotes" in the HTML build
70756b49be4e Merge tag 'docs-6.3' of git://git.lwn.net/linux
c404f5d4f099 docs: Add more information to the HTML sidebar
98e6ab7a0435 bpf: Document usage of the new __bpf_kfunc macro
a33ae832bf3f docs/conf.py: Use about.html only in sidebar of alabaster theme
31abfdda6527 docs: Deprecate use of Sphinx < 2.4.x
e17f22603803 docs/sphinx: More depth in the rtd sidebar toc
bd5d1cc8ffd7 docs: Don't wire font sizes for HTML output
02d33e86468c docs: decruft Documentation/conf.py
2056b920c615 docs: tweak some Alabaster style parameters
d5389d3145ef docs: Switch the default HTML theme to alabaster
05fff6ba04ee Merge branch 'docs-mw' into docs-next
3aa024e4e912 docs: reconfigure the HTML left column
3ef859a4f6c9 Merge branch 'docs-mw' into docs-next
3b384e95642c docs/conf.py: Respect env variable SPHINX_IMGMATH
6b0d3e7c5888 docs/conf.py: Treat mathjax as fallback math renderer
…