As pointed out for underground station, the building outline doesn't always cover the underground levels (i.e. the underground levels can extend far beyond the building limit (and potentially under other buildings/roads...). We find this problem for metro station, train station or other buildings like mall.
One example is the mall "L'Esplanade" (Louvain-la-Neuve, Belgium) that you can see indoor mapped (except parking levels) here https://openlevelup.net/?l=1#18/50.67045/4.61647 . It is on a concrete slab (only pedestrian traffic) that cover all the city center and is more or less 2 levels above the streets where car drive (the streets are in a "tunnel" when going under the city center. Most of the time these 2 levels can be described like "street level / ground level" and the "first level" is for parking, and the "second level" is at the surface with the pedestrian area) and you have building going up to 6-7 floors above that. But for the mall, you have a different leveling : the street level is still the parking entrance, but the two levels above with shops, so that means that the "first level" of shops is below the surface, and indeed it is extended below two plazas and another building. To avoid the mess that could occur with the levels tags, we choose to set an arbitrary level tagging for the whole city center that's on the concrete slab (it can be seen as a large building, and it is not too far from reality). In this case, the level tag need to be synced with all the neighboring building that are built on the same structure (the "concrete slab"), otherwise you would have multiple objects at the same position when rendered on a map. Ex : The mall is officially labelled as "-1" for the parking entrance, "0" and "1" for the shops levels (the level 1 is on the surface), while the building that's on top of some part of the mall have level starting at "0" (the surface level with entrances from the concrete slab) and going up to level 6 above that. If we use these local numbers for the level tag, any tools would render the building surface level at the same position than the shops level that's underneath it !!! For POI it would be like "is it at level 0 in the mall or at level 0 in the building ?", thus an arbitrary level for all these buildings is really needed to avoid such problems. So, i'm really in favor of the level=* for a "data user friendly" tag (that could correspond to local numbering, but not always) and a special tag for the local levels. At this moment i would see a *local level tag *like "level:ref=*" or "loc_level=*" (like we have for name and loc_name ?) but *on each object*, as it would not be realistic to use an outline in the cases i present). PS: i hope my example is not too confusing, or badly explained. :-)
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