In the paper Robot Manipulability they use the symbol (???) after several equations ( (2.5), (2.6), (2.9), ...). Since I've never seen this before, I'm wondering is this should be some kind of notation for e.g. assumption? Or could it be related to the pseudo inverse they use?
Since the paper might be closed-access: Screenshot of (2.5)
The answer is: the question marks have no special meaning, they have to be there without purpose.
Since extended search plus asking a few mathematicians didn't come to any other solution.