cause for access demands (RACH) is coded on 2 octets. this mean it can take up to 256 value (0 to ff in hexa). With NECI “off” not all cause are considered valid (which means not all value for access demand are acceptable) , With NECI “on” all cause are acceptable (even if theory there are still 3 or 4 cause that are reserved for future used). There’s strictly no effect except on phantom Rach. if decoded cause for this phantom is one which was discraded when NECI “off” it will no longer be discarded.