Since I believe you are using PPRNG:
The gender displays what gender the pokemon will be for each possible percentage split between male/female in the following order: 87.5/12.5, 75/25, 50/50, 25/75.
So, in your example of M/M/F/F:
If it's a 87.5% male species, it will be male.
If it's a 75% male species, it will be male.
If it's a 50% male species, it will be female.
If it's a 25% male species, it will be female.
This ordering is slightly different than what's in RNG Reporter (which is 50/50, 87.5/25.5, 75/25, 25/75, I believe), because I found that ordering to be unintuitive.
As for the A and B, it seems it's A is first in and B is second in, unless you're using a ditto, if you go by
this post. I may update the output when doing ditto breeding to be clearer.