landcover

class: More generalized groups of either farmland , ice , wood , grass or wetland . Use this to assign general colors to the landcover.
subclass: Original value of either the landuse , natural or wetland tag. Use this to do more precise styling.

Landcover is used to describe the physical material at the surface of the earth. At lower zoom levels this is from Natural Earth data for glaciers and ice shelves and at higher zoom levels the landcover is implied by OSM tags (like landuse or natural).