Many city centres and even suburban centres have a mixture of land use.
In more densely populated areas it’s quite common to alternate apartment buildings (residential) with office buildings (commercial) with shops on the ground floor (retail). It’s not uncommon for the same building to be used for all three things. Even buildings with light industry in the ground floor routinely contain shops, offices or even apartments.
From reading the land use pages on the OSM wiki, you’d think that this never happens. The land use categories are described as if it were the norm for cities to be neatly separated into business parks, shopping malls, industrial areas and residential subdivisions. There are probably cities like that out there somewhere in the world, but I would think they are the exception.
Many city centres (e.g. London, New York) just aren’t tagged with land use, but from the discussions on the OSM wiki it seems as if the intention is ultimately to cover the entire globe with land use tags. I’d really like to tag my own city with land use because it would be nice to see the city and suburban centres distinguished from purely residential areas when the map is rendered. One thing I could do is pick a category, probably “commercial” or “retail”, and tag the entire city centre with this colour. That’s “tagging for the renderer” though so I’d rather not do that.
The only guidance I’ve found on the wiki seems to be to tag based on "predominant’ use. That helps in some easy cases, but an apartment building with a half dozen shops in the bottom is predominantly used (by volume) for apartments, but more people probably use it for shopping and the building next door could be an office building or a workshop.
So the upshot is that I don’t know how to tag land use in a city centre. I’ve spelled out the issues in detail because there doesn’t seem to be guidance on the wiki which makes me wonder if other people just haven’t encountered this problem. The people who came up with the original classification must have known that city centres exist, so there must be some way that the tags were intended to be used.
Any help would be much appreciated!