Pompey Paul:
Let us try and keep this very simple.
The whole idea of having aircraft flying in controlled airspace separated by a common denominator (such as 1000 ft) is obviously predicated on all such aircraft having the same altimeter setting.
This setting is defined as 1013.25 mbs/hps or 29.92" in the USA and is known as QNE. Such a height when set is described as a Flight Level.
Now then, it has to be established, for very obvious reasons, at what height above the ground it is safe to set such a "false average ICAN standard" setting without making a nasty mess upon a nearby piece of cumulo granitus.
The height at which it is safe to change to 1013.25/29.92" is defined as the Transition Altitude.
How do we arrive at this figure?
It all has to do with the terrain around the airfield in question and the avoidance of collision with said terrain that would be possible before setting altimeters to the new "artificial setting". At the risk of being shot down in flames by the experts (I only went solo 51 years ago) the calculated Transition Altitude would be something like the sum of the highest ground within 25 nms plus 1500 ft plus 10% so that the usual UK airfield would come out to 3000 ft (or more in the frozen north).
So far, so good?
There are of course exceptions to this rule.
In the USA it was decided, many moons ago, that having different TAs for different airfields was probably too much for thick pilots to understand so they took Mt St Helens (before the earthquake), added 1500 ft plus 10% and so came out with a Transition Altitude of 18000 ft which will work whether you are flying out of Miami or Seattle.
Another exception is the London TMA.
So many locals and foreigners flying in and out of LHR, LGW, STN and LTN were failing to reset their altimeters at 3000 ft or thereabouts that it was decided to have a common TA of 6000 ft so that there was just a passing chance that everyone, regardless of race, colour or country of origin sitting in their whiz jets crammed full of whoosh going up at a rate of knots might just manage to get 1013.25/29.92" set before it became very important.
Now then, on the way down, we have to have a safe point at which we have to discard QNE and set QNH otherwise we could have a tangle with the hard bits on the way down having missed them on the way up.
The latest safe point at which to set QNH is known as the Transition Level.
The bit in between (and there has to be bit in between for obvious reasons) is called the Transition Layer.
Now then, our American friends coming off the pond in the early morning quite frequently respond to a descent clearance from London as "XXX Cleared descend 17000 ft". Our tried and trusted UK controllers will always remind them that they are cleared to "FL 170".
It is only when the clearance is to a height that is below the TL that the clearance would be, for example, "XXX cleared descend to 5000 ft QNH 998" would be given.
If you are really still interested, as soon as a clearance below TL is given by ATC, then it is acceptable to immediately set QNH upon the heightimeters.