In the UK the tag ‘ref’ is normally used for roads to state their classification and route number eg M25, A4020, B483 (see examples http://www.openstreetmap.org/#map=14/51.5467/-0.5157)). Recently some other numbers have appeared, such as MC30, UX1. These are presumably numbers used by local authorities in charge of highways but have no meaning to road users; they don’t appear on sign boards or anything else, as far as I know, but are being rendered on OSM in the same way as the official road numbers. This is confusing for users and I feel such numbers should be tagged differently so they don’t render on maps in the same way as M25, A4020 etc.
There are tags like unsigned_ref, official_ref etc which may be more appropriate.
Can you quote some examples, so we can investigate what these ref values might be trying to represent?
Here are two, both entered by the same mapper. I’ve had a closer look. He cites a website which on another page lists the class three roads in Buckinghamshire - https://www.sabre-roads.org.uk/wiki/index.php?title=MC30_(Buckinghamshire) (OSM way 60780241) and the London Borough of Hillingdon - https://www.sabre-roads.org.uk/wiki/index.php?title=UX1_(Hillingdon) (OSM way 134521152).
As I said, unlike the M, A and B numbers, these never feature on roadside route signs so are meaningless to most map users and shouldn’t really be rendered in the same way.
I can see no copyright statements on that site, so I would presume it is not an allowable source for OSM!
I think a lot of people think anything on the web is fair game.