Coincidentally, I read this this morning:
Not from an official source, but a good read nonetheless. https://obversebooks.co.uk/product/49-night-of-the-doctor/
The concept of canonicity, as used by Doctor Who fans at least, is laterally relevant to ‘The Night of the Doctor’s issues of form. Broadly, it is the contested question over the reality or otherwise of fiction: the argument that one fictional story can be more or less real than another fictional story for in-story continuity purposes. It is innately absurd, and derived, albeit at some distance, from Biblical studies12 and (at least in Doctor Who’s case) via the minutiae of Sherlock Holmes fandom13. ‘Canon’ is a noun, not an adjective, and derives ultimately from the ancient Greek ‘Καννα’, a reed used for measuring. To declare something to be part of a canon, whether in the Biblical or Leavisian14 or Holmesian senses, is to count it. Due to its etymological origins a canon can really only be defined from a position of authority15 – something which the authorities behind Doctor Who, e.g. the BBC as an institution or the various people it has placed in charge of Doctor Who, have consistently failed to show any sort of interest in doing, even when prompted.
Not from an official source, but a good read nonetheless. https://obversebooks.co.uk/product/49-night-of-the-doctor/