Martin, What's the difference, in your view, between scratch building and building from a plan? In my view these are the same thing.
The way I am interpreting these at the moment is that the critical thing is what exists when you start work on the boat:
RTR / out of a box? - Boat is substantially operational when you get it. It might be factory made, or by another modeler.
Kit? - Boat comes as a set of parts to be assembled. Most are there, maybe not motor/propshaft/rudder.
Part Kit? - Boat comes as set of major parts, probably including hull and tricky shapes, but some essential simple parts are missing.
Hull only? - Boat hull is bought, perhaps with plan or without, and the rest of the boat has to be obtained elsewhere and made up.
Plans Built? - Boat plans are obtained on their own, and all the rest of the materials have to be obtained elsewhere.
Scratch built? - A modeler wishes to build a model of an existing boat and develops his own plans from the full-size prototype.
Own design? - A modeler has an original idea for a model boat and develops his own plans for it.
But I would guess that, in our lifetimes, we have all done most of these at some point....