Prior to divestiture, numbers that were assigned to offices according to their hierarchical function in the U. S. Public switched telephone network. Note 1: The following class numbers are used:Class 1: Regional Center (RC) Class 2: Sectional Center (SC) Class 3: Primary Center (PC) Class 4: Toll Center (TC) Class 5: End Office (EO) Note 2: Any one center handles traffic from one center to two or more centers lower in the hierarchy. Since divestiture, these designations have become less firm.