Relations in OSM data could be Multipolygon, which represent different kind of land-uses such as lakes, rivers, forest and so on. The question is: there are not tags inside relations which can be used to identify land-uses. But, obviously, the OSM tiles can tell different land-uses. How do they do it?
The multipolygon relation should contain the tags. But there is an old way of making multipolygons, where the relation tags contain only “type=multipolygon”. In that case, the tags are in the “outer” member of the relation (and there should be only one “outer” member).