for info, taken from JAA TGL6
The altitude deviation system will need to signal an alert when the altitude displayed to the flight crew
deviates from selected altitude by more than a nominal threshold value. For aircraft for which an
application for a Type Certificate is made before 1 January 1997, the nominal threshold value will need to
be not greater than ±90 m (±300 ft). For aircraft for which an application for a Type Certificate is made on
or after 1 January 1997, the value will need to be not greater than ±60 m (±200 ft). The overall equipment
tolerance in implementing these nominal values will need to be not greater than ±15 m (±50 ft).