The legal requirement for the Census Bureau to produce subnational population estimates is given in Title 13 of the U.S. Code. In other words, the Census Bureau produces subnational estimates for use in the allocation of funds to state, county and local governments. For this reason, the Census Bureau produces population estimates for general-purpose functioning governments. These governments have elected officials who can provide services and raise revenue. In addition to states and counties, incorporated places and minor civil divisions also serve as general- purpose functioning governmental units.
Counties are the primary legal divisions of most states.
Most counties are functioning governmental units, whose powers and functions vary from state to state. In Louisiana, these primary divisions are known as parishes. The District of Columbia has no primary divisions and the entire area is considered to be the equivalent of a county and in Puerto Rico, municipios are the primary divisions and treated as county equivalents for estimates purposes. Legal changes to county boundaries or names are typically infrequent. These notes also include information on changes to the areas described below.