This has been talked about at length. The IET defines a "circuit" but not a "New Circuit" I would personally consider a new circuits as:-
A circuit for which there is no previous record in the schedule of test results.
However under the 17th Edition 314.1 made it very apparent that the RCD was part of the whole process of splitting every installation into circuits. We test and document the results for the RCD's in a modern consumer unit and this leaves us with a grey area. Where the 2 RCD consumer unit is used would my definition allow one to fit and use a new MCB to feed something not previously used?
Since form 4 does not allow one to group circuits which are all fed from the same RCD but you have to multi-enter the same results using the IET form 4 it would seem my definition would work. But are all forms made that was. Scheme members use different forms so what do the members of this forum think?