>>1663113he means this
B is internal diameter, ID
and it's always given in mm
and A is OD, outside diameter
although both really mean width not diameter.
This actually isn't really something you have to worry about. You'll find people saying what will and won't work but in reality practically everything works. Like the roadie meme now is to match OD to tire width, which is ridiculous, so they're running greater extremes than all the charts would say you shouldn't run.
I think you could happily run even a 1.25" tire on most 26" rims. 1.5" or 1.75" would be the sizes i'd recommend. Paselas come in those size. Get those.
