This data set provides eight 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 six generalized boundaries feature classes and called: ZIPCodes_zp_gen2, ZIPCodes_zp_gen3, ZIPCodes_zp_gen4, ZIPCodes_zp_gen5, ZIPCodes_zp_gen6 and ZIPCodes_zp_gen7. These generalized features classes are provided to be used in mapping applications where very detailed feature classes can slow down performance.