Crosswalk files are important when you try to download census data. For instance, local census financial data are determined by govs_code and the population estimate is provided by fips_code. We are required to use the govs-fips crosswalk file when calculating per capita financial data.
Source: American National Standards Institute (ANSI) and Federal Information Processing Series (FIPS) Codes
Website: https://www.census.gov/library/reference/code-lists/ansi.html
Source: HUD(US department of housing and urban development) USPS zipe code crosswalk files
Website: https://www.huduser.gov/portal/datasets/usps_crosswalk.html