This document is about the detailed layout of the March 2021 Current Population Survey (CPS) Auxiliary data file, which contains identification information, recoded insurance variables, and imputed information on the sources of employer-sponsored insurance. The file is in the same order as the original Census-issued file and can be used with the full person-level CPS file by matching PH-SEQ and A-LINENO. The document provides a comprehensive overview of the variable names, start positions, lengths, and descriptions for the fixed-format file.