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I'm currently studying for my PPL(A) and I've heard this phrase a few times now:
The QNH must be used for flight below the Transition Altitude when flying below Controlled Airspace
RPS should not be used when operating below Controlled Airspace.
Does this mean, you are flying in uncontrolled airspace (class F/G) with Class A-E above you?
So my question is can you have stacked airspaces -- multiple airspaces above a single point?
It seems you're in the UK, and there are 5 different types of airspace (A, C, D, E and G), each of which have different rules about clearances required, minimum visibility etc.
As you have noticed, the airspace above a given point does not have to belong to a single class, but there can be several layers on top of each other. Very common are e.g. the "inverted wedding cakes" of class C airspace above and around airports.
This picture from Germany shows it pretty clearly, the general idea should be the same for the UK:
Most airspace at low altitudes is uncontrolled G except at airports, above that is class E, and only at high altitudes, where IFR traffic (airliners) is predominant, do you have controlled airspace (class C) again. In other countries, this may be class A with no VFR traffic whatsoever allowed (e.g. in the US). In the vicinity of airports you have the control zone (D) and above that, horizontally expanding, different layers of class C airspace, where IFR traffic departs or approaches the airport (the "inverted wedding cake" I referred to).
I am a little surprised that this is not part of what you are studying, but it may depend on the order in which you approach the different subjects.
