Interesting discussion - although it does rather appear to be getting heavily bogged down with the minutiae.
As an outsider looking in I sought out how the NMRA approaches competitions and being a non-S7 member you can call me all the names under the sun as this would appear to be none of my business
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Basically the NMRA have two categories
scratch built and kit built viz:
"a. Scratch Built - This class is intended for models built largely from basic or scratch materials."
"b. Kit Built - This classification is intended for models comprised of more than 50% kit or largely commercial parts to allow them to compete with similarly built models rather than typically higher scoring scratch built models."
NMRA judging is based around five areas:
Construction - focusing on the
quality of workmanship.
Detail - how many parts and complexity.
Conformity - how well does the model reproduce the prototype?
Finish and Lettering - complexity of finish and lettering and the quality of skill of it's application.
Scratchbuilding - how much was scratchbuilt.
As I see it only two categories would be required. Scratch Built - as it's title implies; and Kit Built - which would naturally include kits (where 50% or more of it is used) and modified RTR. The skill is in the judging
The entry would be supported by a brief description by the entrant outling their approach including whether the entry was professionally painted and lettered.
To end, in my view 3D printed models would be classed as kit built as you are starting with a 'finished' product and still have to add wheels, couplers, paint finish, decals and other detail parts where necessary.