A git commit is composed of a number of headers (key: value fields) and a commit message.

There is a set of "standard headers" (tree, parent*, author, committer, encoding?), but then you can add more. In fact there's a set of semi-standard headers, as in headers git itself will add under some conditions: `gpgsig` and `gpgsig-sha256` if the commit is signed, and I think `mergetag` for signed tags. They are documented as part of the signature format but they're not "baseline" features: https://git-scm.com/docs/signature-format#_commit_signatures

But because of this, a git client should support arbitrary commit headers, and round-trip them even if it does not expose them.