This requires many judgements. Designing map units to indicate significant differences in behavior among soils is particularly important to meet the current objectives of a survey. Reflecting differences in genesis and morphology is also important, even if no immediate differences in interpretations are known. A potential benefit must be weighed carefully against the costs incurred in making additional separations. One objective of every soil survey is to record knowledge about soils, but this does not mean that the soil map must show the location of every kind of soil in a survey area or that the publication must record all that has been learned about the soils.
Taxonomic classes provide the basic sets of soil properties with which soil map units are defined.
They summarize an immense amount of research and experience related to the significance of soil properties and combinations of properties. They provide predefined sets of soil properties that have been tested for genetic relationships and for interpretative value. Taxa provide a firm base for recognizing the components of potential map units in an unfamiliar area. Using established taxa is much easier than independently sorting out sets of properties and determining significant class limits. The objectives of a survey determine the kind of map units and the taxonomic level used to identify components of map units.