Clive, it's pretty simple really. If it qualifies for ACSC Catalogue status it is CTO, anything else is per favour. That is my opinion, in any case.
As I said, Peter, I agree. That doesn't alter the fact that the term CTO has a much wider use.
I perhaps didn't make myself clear - a common failing these days as I hurtle into my dotage. The confusion out there is obvious. Paul's post about his CTO block is good case in point, as for that matter was our Revered Leader's 5d green plate block and Traralgon's 5d QE2.
A few pages back on this thread, Rod posted a picture of the best block of 4 of the KGV 1d violet anyone is ever likely to see, complete with 'correct' CTO cancellation. But it can't, by 'ACSC catalogue' definition be CTO - I wonder if I should put a question mark here. Yet, if, horror of horrors, Rod split that block into four single stamps there wouldn't be a person on the planet who wouldn't agree that the stamps were CTO from a collector's pack. Same story with Traralgon's block.
And as for a fad in the 1950s and 1960s for CTO' blocks of 4 - yes, absolutely, there was such a fashion, as any amount of material still around bears witness. Very, very popular at the time. And they were invariably described as cancelled to order. Of course, in those days, such stamps were merely listed as used in Australia's foremost catalogue, as were 'pure' CTOs.
It is only in relatively recent years - perhaps as late as the 1990s (I no longer have my 1983 loose leaf ACSC) that the somewhat arbitrary catalogue distinction has been made between CTO and 'used' (including per favour), and that by just one (if authoritative) catalogue editor.
Just musing, mostly.