Transition level
Surprised that nobody has yet mentioned the reasons for sometimes using altitude (terrain separation) and sometimes flight levels (worldwide standard pressure setting 1013.2 = 29.92 in Hg = 760 mm Hg).
Hence most states have set their tranisiton altitude to be slightly above the highest terrain in that FIR (Flight Information Region), in the case of Australia the highest LSALT is 8400' so TA at 10,000' allows at least one IFR and one VFR altitude in each direction over all of the country. Similarly in UK most of country is on TA of 3000' (though Snowdon 3560' amsl is also in this area) but Scottish FIR has TA 4000' (Ben Nevis 4406' also above it - but gives about 500' clearance between the FL that is 1000' above TA and the ground if the pressure is at or above 1013.
International convention gives us that the lowest usable IFR level must be at least minimum IFR separation (nominally 1000') above the transition altitude. Transition level is usually the nominal 1000' level above TA, but may not be usable if the QNH is below 1013 .
The USA is different because they have chosen to use altitude up to a level of FL 180, where they change airspace classification to Cleaa A (IFR only allowed to cruise there) - so the logic is different. Some pacific island states use 9500' as the TA because they have inherited that as the highest level that unpressurised VFRs can use, so everything pressurised will use FLs.
One of the responsibiliites of ATC when they have traffic approaching a boundary is to co-ordinate with the neighbouring sector to ensure vertical separation - just as important for any sector change, but more complex when there is a change in TA/TL or when (e.g. Russia and China) one sector uses 500 metre separation and metric altimetry, whilst the other side uses good old feet and flight levels.
Got it? Pity poor old Tibet, where FL 290 is LSALT on some routes across the Himalayas and the 747s from SIN and BKK to FRA and LHR don't have a big choice of levels due to traffic density on the route.
But it all beats the noise of aeroplanes bumping into each other!
Wombat too