The MapUnits feature class contains polygon corporate data which depicts legacy Forest Service Soil Resource Inventory (SRI) soil map units in USDA Forest Service Region 6.

The SRIs were originally compiled in 20 volumes, with the original year of publication ranging from 1969 to 1979. The Gifford-Pinchot SRI was redone following the eruption of Mt Saint Helens, and that version was published in 1992. The Olympic NF also produced two versions, the original version being published in 1969, and an updated version in 1982. This regional compilation contains polygons from the original 1969 Olympic SRI and the 1992 Gifford-Pinchot SRI. The Colville National Forest was the only Region 6 forest that did not compile a SRI.

All of the original SRI volumes were published prior to the availability of commercial GIS software (ARC/INFO, for example, was first released in 1982). However, some forests subsequently digitized their SRI polygons, while some had not. Those SRIs that had not been digitized by the forests were subsequently digitized by the Oregon State University Crop & Soils Department. Some forests had digitized their SRIs, but had subsequently modified the line work from the original maps. In these latter cases, OSU re-digitized portions of the line work obtained from the forest to match the original delineations. No edge-matching between individual forest SRIs has been done.

A soil "Map Unit" is the basic spatial unit for which soil properties are mapped. Map units in the SRIs were described as either "pure" map units, for which the described soil generally occupies 70% or greater of the map unit (polygon), or as a soil "complex", which is comprised of two or more soil types in given areal percentages of the map unit. In a complex, the spatial extents of the individual soil components are not mapped, though tabular soil properties are recorded for each soil component.

The attribute table for the layer essentially contains just the soil map unit code; specific soils information is recorded in ancillary tables that relate to the polygons by the map unit code. There is a many-to-one relationship between the MapUnits feature class and the various soil properties tables (i.e., there are potentially many polygons across the landscape that have the same map unit number, meaning that the same described soil may occur in many places across the landscape).