Are differential backups backing up only files that changed or are they backing up partial data (diff) of the files that changes?

[Fire-Dragon-DoL]

Forgive the question, I couldn't find the answer.
Differential backups perform a "diff" in my mind.
If a file has changed, will the backup contain only the part of the file that has changed, or will it contain the whole file, but only the files that did change?

[mtkennerly]

Hi!

contain the whole file, but only the files that did change

This is what Ludusavi currently supports as a "differential backup".

the backup contain only the part of the file that has changed

Ludusavi doesn't support this, but it's usually referred to as an "incremental backup".