I think hand size, but I honestly have no idea. I just notice that this is what I ordered . PM me if you do want them and we can coordinate the details.
Stopped in today and picked up the 105 brifters and RD. Payment (what little there was of it) is in the box.
Will be the better part of the drivetrain upgrade on one of the pedawans bikes. the ace 650 come with bar mount sort of paddle shifters. They work well for smaller hands and less strong arms but both kids are moving past that need rather quickly.