The CT00 layer contains polygons for census areas. The source for the information in this layer is the U.S. Bureau of the Census and appropriate county government agencies.
This file is extracted from the Los Angeles County Registrar/Recorder's Precinct Information File, which is designed to match the Thomas Brothers TRNL (road network) layer.
Where census boundaries are defined by streets, water, city boundaries, or other features, those arc segments are copied to the CT00 layer, so that census tracts and other layer features match perfectly.