Under what circumstances is the Mach Number Technique (MNT) applied by Area Control Centres (such as BACC) to maintain longitudinal separation on en-route routes?

1. Between propeller-driven aircraft flying at low altitudes below 5,000 ft
2. Only during low-visibility ground taxi operations
3. Between helicopters operating within the CTR
4. Between turbojet aircraft on the same route or diverging routes, where aircraft maintain assigned Mach numbers to prevent closing speed