This is used in procedural environment. If we start by looking into provision a) with the following example: Arriving aircraft is 7 minutes from the threshold of runway 29 with descend to 5000ft. Departing aircraft is cleared for take-off on runway 11 to 4000ft.
Provision a) allows for opposite departures but how is vertical change possible?
My understanding is that the departing aircraft can take-off below the inbound aircraft but another form of separation is required to climb through the arriving aircraft.