GIS Match Codes

I’m struggling to understand what these match codes match to-- zip codes? FIPS? Where can I find documentation to interpret them as geographic locations.
Thanks in advance.

The geographic codes in the microdata available via IPUMS Terra correspond to the data available via IPUMS International. So, for example the GEO2_US2005variable indicates the PUMA (or public use microdata area), which is the second level of geographic identification available in US samples. A complete listing of all geographic variables available in IPUMS International (and by extension IPUMS Terra) is available here.