(n.) A condition in which two or more different keys of the same level of keying and under the same higher level key(s) operate one cylinder by design; e.g., XAA1 operated by AA2 (but not XAA1 operated by AB1).
Note: This condition could severely limit the security of the cylinder and the maximum expansion of the system when (1) more than a few of these different keys operate a cylinder, or (2) more than a few differently cross keyed cylinders per system are required.

Source: Lock Industry Standards and Training (LIST) Council