In most cases, Subregional Study Areas correspond to Local Agency Formation Commission (LAFCO) Spheres of Influence (SOI). LAFCOs influence how certain types of governmental responsibilities are divided among jurisdictions and service districts within a county. LAFCOs have jurisdiction over city incorporations, the formation or consolidation of special districts, and the annexation of territory by cities and districts. LAFCOs designate urban service district boundaries as well as SOIs for cities.

An SOI represents "the probable ultimate physical boundaries and service area of a local agency." As land outside a city's limits, but within its sphere, becomes urbanized, residents of the area generally seek annexation in order to receive urban services. Thus, the incorporated limits for many cities change with relative frequency. The Metropolitan Transportation Commission uses the LAFCO SOI to define most subregional planning areas in an effort to present data that can be compared over time for areas that have consistent boundaries.

Other subregional areas in the feature set represent one of the following: a proposed city LAFCO sphere, the city and part of its LAFCO sphere (e.g., the city of Napa's subregional study area is defined by the City's Rural Urban Limit, which excludes portions of its LAFCO sphere), or unincorporated rural areas with significant concentrations of population, households, or employment.

In seven counties (all except San Francisco and Sonoma), the County "Remainder" area refers to the remaining unincorporated territory outside all other subregional areas.