The fixed cup is the one screwed into the drive (right) side of the BB; the adjustable cup is on the non-drive (left) side. It's that way so you can get at it to adjust bearing tension with the crank arm off while the chain is in place on the other side (if it were the other way around you'd have to take off the chain and chainwheel crank). The spindle length is the distance in mm between the left and right spindle bearing races (surfaces).