AFAB = Assigned Female At Birth
AMAB = Assigned Male at Birth
Basically not all men or women are necessarily cisgender, and some people are nonbinary, so you might have AFAB trans men whose height or skeletal structure was impacted by that, and nonbinary people may not necessarily consider themselves "men" or "women", but their build may be impacted by their hormones, chromasones and stuff, and it's more accurate and respectful to say what their assigned gender was at birth (where the doctor just kinda looks at what the genitals look like and calls it how they think it looks), than to say reductive, and often inaccurate things like "biologically male" or "biologically female" (because even if somebody looks pretty male or female at a glance, you never actually know what's going on with their chromasones and things)..
It's another reason really to just draw characters as individuals rather than trying to split things too much along lines like age and gender.