sorry - I meant the top part of the bottom u-joint.
this would be while removing the steering rack.
unscrew the tightening bolt on the upper sleeve, spread the upper sleeve, then take the u-joint off the steering shaft - it would come off the car with the rack.
the slot pattern on the upper sleeve (still talking about the lower u-joint here) is, to me, easier to line up and mount than the pattern.on the lower sleeve. so it's easier when replacing a rack to let the disconnection/reconnection point be at the upper sleeve.

I've never tried to replace a u-joint without removing the rack.
seems it would be difficult to move the steering shaft upwards enough to disengage a u-joint.
likewise it might be difficult to get enough play in the hoses and tie rods to drop the rack enough to disengage that way.