Reading the above stuff does my head in
The answer must surely depend on whether one is trying to
1) answer an exam question (in which case I can't help because it will be heavily context and airspace/country dependent) or
2) is trying to fly safely.
On the 2nd one:
When outside controlled airspace, you can legally fly anywhere, at any height (subject to the 500ft/1000ft etc minimum rules etc) but fairly obviously you have to set the altimeter to either 1013mb (29.92") or the local airspace owner's QNH, according to whether the base of CAS is specified as feet or as a flight level.
If under an ATC service, and this includes flight in controlled airspace, you fly at a level/altitude and with an altimeter setting
as negotiated with ATC.
Personally, when OCAS, I fly at "funny" levels e.g 3700ft because one is less likely to bump into somebody there. I would never fly at 2000ft or 3000ft for any length of time. But when approaching CAS and about to ask for a transit, then I sort myself out at one of these round numbers, to stop ATC wondering what the hell I am doing flying at 3700ft. So I might approach e.g. Solent doing my best to look good, at FL040.... only to have them grant me a transit at 4000ft on a QNH of XXXX
Like I say, you do whatever ATC would like you to do, and te rest of the time you do what you want to do.
Shall we start a thread on the minimum cloud separation, in different classes of airspace, in different countries, versus day/night?
Oh and I nearly forgot... versus VFR/IFR flight.