PUMA populations, 100,000 or 65,000?

I think I got it!

The 65,000 threshold is for the tables.  They include only areas with >= 65,000, otherwise the area is not listed

The 100,000 threshold is for PUMAs.  These come into play while using the PUMS data

PUMAs appear in both the ACS PUMS and the ACS summary tables.

ACS summary tables are provided for many different summary levels: counties, census tracts, ZIP Code Tabulation Areas, and many others, including PUMAs.

Every PUMA must include 100,000 residents (in the year of its reference census, e.g., each PUMA in the 2020 PUMA definitions included at least 100,000 residents according to the 2020 census).

ACS 1-year summary tables cover only areas that include at 65,000 residents. That includes all states, many but not all counties, and all PUMAs.

ACS 5-year summary tables cover all areas in each summary level covered by a given table.

PUMAs are the only sub-state areas identified in PUMS. PUMAs have a many-to-many relationship to counties. I.e., it may be possible to crosswalk from a single PUMA to a single county, or from multiple PUMAs to a single county, or from a single PUMA to multiple counties; it varies. But counties are not directly identified in the PUMS when accessed directly from the Census website.

If you get ACS microdata via IPUMS USA, then you can also get identifiers for counties, cities, or metro areas where it's possible to identify them based on the identified PUMAs (accepting a small degree of mismatch between the PUMA extents and the city or metro areas).

PUMAs appear in both the ACS PUMS and the ACS summary tables.

ACS summary tables are provided for many different summary levels: counties, census tracts, ZIP Code Tabulation Areas, and many others, including PUMAs.

Every PUMA must include 100,000 residents (in the year of its reference census, e.g., each PUMA in the 2020 PUMA definitions included at least 100,000 residents according to the 2020 census).

ACS 1-year summary tables cover only areas that include at 65,000 residents. That includes all states, many but not all counties, and all PUMAs.

ACS 5-year summary tables cover all areas in each summary level covered by a given table.

PUMAs are the only sub-state areas identified in PUMS. PUMAs have a many-to-many relationship to counties. I.e., it may be possible to crosswalk from a single PUMA to a single county, or from multiple PUMAs to a single county, or from a single PUMA to multiple counties; it varies. But counties are not directly identified in the PUMS when accessed directly from the Census website.

If you get ACS microdata via IPUMS USA, then you can also get identifiers for counties, cities, or metro areas where it's possible to identify them based on the identified PUMAs (accepting a small degree of mismatch between the PUMA extents and the city or metro areas).

Thanks Jonathan

Your paragraph

"PUMAs are the only sub-state areas identified in PUMS. PUMAs have a many-to-many relationship to counties. I.e., it may be possible to crosswalk from a single PUMA to a single county, or from multiple PUMAs to a single county, or from a single PUMA to multiple counties; it varies. But counties are not directly identified in the PUMS when accessed directly from the Census website."

states it very well!!

I will take another look at IPUMS.  Thank you for the information