AFAB
Assigned female at birth — a fact about a birth certificate, not an identity.
AFAB means a doctor recorded female on the paperwork when someone was born. That's the whole of it. It describes a moment of classification, not who a person is.
It's useful in specific contexts — talking about medical history, or about the particular social treatment someone received growing up. It is not a polite way to say a trans man is really a woman, and using it that way is why many trans people have grown wary of it.
The rule that keeps it respectful: AFAB is something that was done to a person, never something they are. A trans man is a man who happens to be AFAB, and mostly that second half doesn't need saying.