Can anyone tell me what this means, and what 'A Pass' and 'B Pass' are?
I only read a vague explanation about it, but it literally just grades how well the serve receive is
An A Pass is more or less passed just above the setter's box, wherein they don't have to adjust or chase the ball that much, it's usually when they try to pull quicks and combination plays.
A B Pass is around 1-2 meters away from the setter's box, not that bad, just gives the setters fewer options, unless they can pull off good quicks with underhand sets, run or kneel while setting and whatnot (I think I remember Miyashita doing some of these during the Rio era). Some of them are still salvageable to the point where a setter can still set up quicks and combination plays though.
There's also a C Pass = more than 2 meters away from the setter's box, usually when the setters are forced to use the pins (I categorize them as Out of System Plays though).