[Tagging] 'Unknown' value.

[OSMDoudou]

Others already gave possible causes, so I won't add or repeat.

It's pointless tagging and leaves us guessing whatever it could mean.

If your analysis shows these tags don't add value (this is to say: don't make a gigantic update on places you haven't reviewed), you better remove them (possibly with a comment on the changeset explaining you're removing tags which don't make sense).