Understanding error output when a subkey expires

[jman]

I use GnuPG 2.4.7 on Debian/trixie. I recently renewed a set of subkeys but forgot to update the 
signing key in my git config, so commits failed with a message like this:

--8<---------------cut here---------------start------------->8---
$ git commit -m 'commit description'
error: gpg failed to sign the data:
[GNUPG:] KEYEXPIRED 1234567890
[GNUPG:] KEY_CONSIDERED <fingerprint> 3
gpg: skipped "0xDEADBEEF": Unusable secret key
[GNUPG:] INV_SGNR 9 0xDEADBEEF
[GNUPG:] FAILURE sign 54
gpg: signing failed: Unusable secret key
--8<---------------cut here---------------end--------------->8---

The only part I understand is "KEYEXPIRED" and "skipped "0xDEADBEEF": Unusable secret key", the rest 
of the output is not very helpful and web searches seem to not lead to information/documentation.

Specifically, I would like to understand these cryptic error messages:
- [GNUPG:] KEY_CONSIDERED <fingerprint> 3 (what does the 3 stand for?)
- [GNUPG:] INV_SGNR 9
- [GNUPG:] FAILURE sign 54

is there any documentation?

Thanks.