“Region, Division, State, and PUMAs are the only geographic areas identified in the ACS PUMS” (Census Website). But on your website (IPUMS), PUMS data are available on county level as well. Why? Thank you!
As is detailed on the COUNTY variable documentation:
Counties are unavailable in public-use microdata from 1950 onwards. However, it is possible to recover some counties from low-level geographic identifiers. These include State Economic Areas (SEA) in 1950; county groups in 1970 (CNTYGP97) and 1980 (CNTYGP98); and Public Use Microdata Areas (PUMA) from 1990 onwards, including Super-PUMAs (PUMASUPR) in 2000 (COUNTY cannot be constructed for 1960 1 % because geographic information below the state level is not currently available). Counties were identifiable if: (a) they were coterminous with a single identifiable SEA, county group, or PUMA; or (b) they contained multiple identifiable SEAs, county groups, or PUMAs, none of which extended into other counties.