The US FWS has designed the various levels of this classification for specific purposes, and the relative importance of each will vary among users. The Systems and Subsystems are most important in applications involving large regions or the entire country. They serve to organize the Classes into meaningful assemblages of information for data storage and retrieval. 


The Classes and Subclasses are the most important part of the classification for many users and are basic to habitat mapping. Most Classes should be easily recognizable by users in a wide variety of disciplines. However, the Class designations apply to average conditions over a period of years and, since many habitats are dynamic and subject to rapid changes in appearance, the proper classification will frequently require data that span a period of years and several seasons in each of those years. 


The Dominance Type is most important to users interested in detailed regional studies. It may be necessary to identify Dominance Types in order to determine which modifying terms are appropriate, because plants and animals present in an area tend to reflect environmental conditions over a period of time. Water Regime can be determined from long-term hydrologic studies where these are available. The more common procedure is to estimate this characteristic from the Dominance Types. Several studies have related water regimes to the presence and distribution of plants or animals (e.g., Stephenson and Stephenson 1972; Stewart and Kantrud 1972; Chapman 1974). 


Similarly, we do not intend that salinity measurements be made for all wetlands or deepwater habitats except where these data are required; often plant species or associations can be used to indicate broad salinity ranges. Lists of halophytes have been prepared for both coastal and inland areas (e.g., Duncan 1974; MacDonald and Barbour 1974; Ungar 1974), and a number of floristic and ecological studies have described plants that are indicators of salinity (e.g., Penfound and Hathaway 1938; Moyle 1945; Kurz and Wagner 1957; Dillon 1966; Anderson et al. 1968; Chabreck 1972; Stewart and Kantrud 1972; Ungar 1974; Odum et al. 1984). 


In areas where the Dominance Types to be expected under different Water Regimes and types of Water Chemistry Modifiers have not been identified, detailed regional studies will be required before the classification can be applied in detail. In areas where detailed soil maps are available, it is also possible to infer Water Regime and Water Chemistry from soil map units. 


Some of the Modifiers are an integral part of this classification and their use is essential; others are used only for detailed applications or for special cases. The minimum standard for wetland classification is: System, Subsystem (with the exception of Palustrine), Class, Subclass (only required for Forested, Scrub-Shrub, and Emergent Wetland Classes), Water Regime Modifier, and Special Modifier (only required where applicable). The minimum standard for deepwater habitat classification is: System, Subsystem, Class, and Water Regime Modifier. Water Chemistry Modifiers and Soil Modifiers generally are not used when classification data are obtained using remote sensing. These Modifiers should be applied only when detailed, supporting data have been gathered in the field or from reliable sources such as soil surveys. 


The user is urged not to rely on single observations of Water Regime or Water Chemistry. Such measurements give misleading results in all but the most stable habitats. If a more detailed Soil Modifier, such as soil order or suborder (Soil Survey Staff, 2010) can be obtained, it should be used in place of the Modifiers, Mineral and Organic.

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