This part is inserted into the ZX9R CVR Air Correction Jet inlet preventing the jet from operating. This is a common modification for these carbs when fitted to car engines. (it's really difficult to get them to provide the right amount of adjustment to the fueling when used for cars).
The part pushes into the port and is held in place by the carb mating part. No drilling, tapping or glue-ing is required and it can just be pulled out if needs be.