(My changeset message here wasn't great, sorry).

addr:flat isn't a real tag, it has very imitated usage globally, literally 290 as of 2025-10-31 00:59 UTC. And is long discouraged on the wiki.

https://taginfo.openstreetmap.org/search?q=addr%3Aflat

https://wiki.openstreetmap.org/wiki/Key:addr:flats#Similar_tags

addr:flats is a legitimate tag used globally, it is specifically documented in GB for ranges of flats. It should probably be "Flats 5-12", not "5-12".

addr:unit is legitimate tag used globally for a single unit (or flat/apartment/suite) hence why it's necessary to add the prefix to the value.

https://wiki.openstreetmap.org/wiki/Addresses_in_the_United_Kingdom#Address_tags

So addr:flats for range, addr:unit for single. Confusingly named, but relatively simple definitions.