This data set provides four feature classes. The base feature class is called ZIPCodes_zp and isn't generalized. The weighted centroids feature class is called ZIPCodes_zp_cent. The centroids are weighted by the U.S. Block Centroids population distribution. Use the weighted centroids in report aggregation and spatial overlay operations. The ZIPCodes_zp and ZIPCodes_zp_cent feature classes contain all the attributes. There are two generalized boundaries feature classes and called: ZIPCodes_zp_gen2, and ZIPCodes_zp_gen3. Use the generalized boundaries when creating study areas.