Grey Area
I am not qualified offer an opinion on this, but it has a parallel with Area Control.
You are cruising at FL370 and I clear you to FL270...sometimes I may say "descend" FL270.....a common response to the latter is "now or at pilot's discretion?"
In earlier years there was a difference but this has been eroded by time. Now I have to add a qualifier, such as "descend now" or "cleared when ready".
One is an executive instruction, one is a clearance [permission].
So when you are issued with a ground movement clearance do you pilots believe that you have been given a clearance to do something i.e. at your discretion/when ready ..or do you believe that you have been given a mandatory instruction i.e. compulsory.
The meaning of ATC words should not go through a transformation depending on whether you are in the air or on the ground.
If a pilot is "cleared" to a certain holding point, does he have to go all the way up to that point without delay?