I know that there are four different types of production models, including mass-production, limited production, prototype, and custom. But what about one-off models? Why isn't there a one-off production type among the selection of production types?
I see your point and agree. Most of Gundam from G are intend to be unique unit (sure, their data will be used to produce the next one, but that isn't its current purpose), same as the first five Gundam from W.
Although I wonder how come prototype is count as production type? It's purpose of the unit, not indicate its quanity.
I don't think the types are exclusive. I would say the RX-178 would be both Prototype and Limited Production, since 3 of them were built, and it was specifically built for developmental purposes. I would say that if a fully functional mobile suit prototype is produced it may be considered limited production, even if the number of production models is only 1. The MSN-001 Delta Gundam would simply be a prototype mobile suit and nothing else, since schematics exist as well as simulations, but it was never actually built.
Custom is also a tricky one to define, since it could mean it is a modified version of an existing suit, or it could mean that it was a suit built to the pilot's specifications. Or would you consider a suit that only gets produced once a custom suit?