I agree with what you say about the semantics, but that doesn't really help us with the sort of issues/questions arise (primarily relating to 'notifiability').If a circuit is defined as a "Final circuit" (A circuit connected directly to current - using equipment, or to a socket-outlet or socket-outlets or other outlet points for the connection of such equipment.) then how can it feed any other circuit? It is like saying very end. Clearly very is not required as end is the end. Final is in the same way the last circuit you can't have the very final circuit in same way as can't have very end or very last.
It's all very well saying that 'a final circuit' is 'final' but what then happens if one then extends it to feed additional loads via a further OPD(s) (FCU, MCB in a CU or whatever). Does that first circuit now cease to be a 'final circuit'? If so, what do you now call it - as far as the downstream OPD is concerned, I suppose you could call it a 'submain' - but that would make very little sense if it were primarily a circuit supplying, say, a number of sockets.
As has been extensively discussed, probably the greatest problem in attempting to define 'a circuit' (hence 'a new circuit') is finding a definition that does not result in an FCU creating a 'new circuit'.
Kind Regards, John