Soils differ in the size and shape of their areas, in their degree of contrast with adjacent soils, and in their geographic relationships due to soil formation or land use. Soil surveys use four kinds of map units to distinguish the different relationships: consociations, complexes, associations, and undifferentiated groups.
Consociations
In a consociation, delineated areas use a single name from the dominant component in the map unit. Dissimilar components are minor in extent. The soil component in a consociation may be identified at any taxonomic level. Soil series is the lowest taxonomic level. A consociation map unit that is named for a miscellaneous area is dominantly that kind of area and any minor components present do not significantly affect the use of the map unit.
Complexes and associations
Complexes and associations consist of two or more dissimilar components that occur in a regularly repeating pattern. The total amount of other dissimilar components is minor in extent. The following arbitrary rule determines whether "complex" or "association" is used in the name. The major components of a complex cannot be delineated separately at the scale of mapping. The major components of an association can be delineated separately at the scale of mapping. In either case, because the major components are sufficiently different in morphology or behavior, the map unit cannot be called a consociation. In each delineation of a complex or an association, each major component is normally present though their proportions may vary appreciably from one delineation to another.
Undifferentiated groups
Undifferentiated groups consist of two or more components that are not consistently associated geographically and, therefore, do not always occur together in the same map delineation. These components are included in the same named map unit because their use and management are the same or very similar for common uses. Generally, they are grouped together because some common feature, such as steepness, stoniness, or flooding, determines their use and management. If two or more very steep soils that are geographically separated are so similar in their potentials for use and management that defining two or more additional map units would serve no useful purpose, they may be included in the same unit. Each delineation has at least one of the major components, and some may have all of them. The same principles regarding the proportion of minor components that apply to consociations also apply to undifferentiated groups.
Source: U.S. Department of Agriculture, Natural Resources Conservation Service. National soil survey handbook, title 430-VI. Available online. Accessed 9/17/2012.