In addition to political and geographical boundaries, a large amount of work has gone into developing usable boundaries based on classifying various items of interest. For example, this passage from Bailey (1976) explains the thinking for "Ecosystems of the US":
"To manage forest, range and related land on a national, regional and local scale, requires a classification system that is objective, that covers the whole country, and that is hierarchical in nature. To be objective, the system should be based on observed properties. The same method of classifying land in California should be used in Maine and in Florida. The system should be hierarchical, allowing for both broad levels of generalization and highly specific levels, which would allow for aggregating data at different levels to meet decision-making needs at regional, state, and national levels."
Many classification systems exist. This section attempts to present the most notable from a national and regional level, and offers suggestions on selecting the most appropriate system for your particular use.