I don't know how much stock the experts here put in wikipedia, but
the definition of repeater over there essentially says "any non-single-shot firearm that can fire more than one shot out of a single barrel and readies the gun for the next shot after firing." Wikipedia also goes on to include everything from lever-action to bolt-action to pump-action to gas operation, et. al., under the term "repeater."
So extrapolating from this definition, pepperbox pistols and nock/volley guns probably wouldn't count as repeating firearms (because you're just discharging multiple single-shot barrels), but not much else outside of the pre-modern age of firearms can be said to not be a repeater. That's why I'm unsure of including a repeater category since according to what I know it's too broad, like the "carbine" category.