designations that serve as guides to advertisers and advertising agencies in the preparation of media plans, first used by the original ACNielsen Company , to indicate counties in the United States by size of population (based on the most recent Census) and proximity to a central city or metropolitan area. All counties in the United States have only one designation.
A County: any county located in the 25 largest U.S. cities or their consolidated statistical urban areas.
B County: any county not designated as an A County that has population over 150,000 or is part of a consolidated statistical area with population over 150,000.
C County: any county or consolidated statistical area not designated as an A or B County that has population over 40,000.
D County: any county statistical area not designated as an A, B, or C County.

The consolidated areas are determined by the Federal Government Office of Management and Budget and are considered to be standard.