The object WRB classifies as soil is: "Any material within 2 meters from the Earth's surface that is in contact with the atmosphere, with the exclusion of life organisms, areas with continuous ice not covered by other material, and water bodies deeper than 2 meters ”. The definition includes continuous rock, paved urban soils, soils of industrial areas, cave soils as well as sub-aqueous soils.

WRB is a two tier system: TIER 1: The Reference Soil Groups Thirty-one reference soil groups at the first categorical level

TIER 2: The combination of Reference Soil Groups with Qualifiers Consisting of combinations of a set of unique qualifiers added to the reference soil groups