weak or mini-NT
2C = "natural"
I think its general bridge knowledge that 2C can be phony, sometimes you have a 3card fragment and try to avoid the axe (hope opener got 4 or 5 clubs) or sometimes you just want to XX until opener play the hand (like with long S or both majors).
However if you have the agreement that 2C followed by a XX is your way to show a very weak 3 suiters short in clubs (4441,5440,4540,4450) should 2C & the XX be alerted ? Just to be sure im not describing that 2C followed by XX will always show a 3 suiter but that very weak 3 suiters short in clubs are systematically under 2C followed by XX.
For the poll Im looking for the ACBL point of view. But for WBF or others feel free to comment.
The EBU blue book got ...
4h3d A bid of two of a suit by responder when an opening 1NT has been doubled, if it is ostensibly natural but there is a possibility that responder will remove or redouble for take out: this is considered general bridge knowledge.
Plus... it's free!