Perhaps it would help to re-ask the question slightly differently. Taking Leeds as an example, there are a bunch of different sorts of cycle routes that run through here. To my knowledge, there are no international routes, but there are several national routes. these are all part of Sustrans’ network, and the caveats above apply - there are gaps in them so you can’t follow them from one end to the other.
There’s also the National Byway - a national route tagged as an network=rcn in the UK, There’s also the West Yorkshire Cycle Route, which is a genuine regional route, and signed as such.
Also signed as rcn are Leeds’ “cycling superhighways” CS1, CS2, and (in OSM) CS3. On the ground signage for these in the centre is challenging (compare nodes and ways there), but as you get further out it’s simpler.
Finally, lots of things mapped as lcn, including a couple of fairly large ones such as the Wakefield Wheel.
Clearly something the CS1 and CS2 are “a bit like trunk roads”. Following that analogy, the WYCR and Wakefield Wheel are “primary road bypasses of a major town”, and the National Byway is more like the Wild Atlantic Way or the North Coast 500.
Perhaps in addition to “i”, “n”, “r” and “l” we need something that says “who it’s for” - hyperlocal or further-afield commuters, Strava-fans or out-and-out tourists.