The District of Columbia is considered its own county. These may be variously called townships , boroughs , or districts of various kinds. In other states, though, the County Subdivisions have no legal existence or governmental function, and are called Census County Divisions. In those states, the County Subdivision summary levels can pretty much be ignored. A County Subdivision of whatever kind will have a five-digit FIPS code.
Every state has a two-digit FIPS code, assigned in alphabetical order.
Generally, you won't need to pay much attention to either of these levels of geography. Because Place boundaries can cut across Census Tracts and even across County boundaries, Places are not part of the tiling hierarchy. However, the boundaries of CDPs have no legal status, and are simply defined by local officials for statistical data-gathering reasons. A good example of the usefulness of CDPs can be seen in South Florida. More than half of the population of Dade County lives outside the incorporated areas like Miami and Hialeah.