The state a county FIPS code is assigned within.

A county FIPS is five digits: the two-digit FIPS of the state that assigns it, then a three-digit county code. 51107 is Loudoun County, and the 51 in front of it is Virginia. That prefix is the only statement of a county's state either SimpleMaps file makes that cannot disagree with itself -- a row's own state_id column describes the row, not the county, and the two differ wherever a mailing address crosses a state line. ZIP 20041 carries state_id DC and county_fips 51107: Dulles-area mail with a District of Columbia address, delivered to a county in Virginia. 150 of the source's 3,233 counties are named on rows of more than one state.

So the mapping has to run the other way -- from FIPS to state, not from the row -- and neither file carries it. uscities and uszips name states only by their two-letter postal code, never by state FIPS, and no row anywhere states that 51 is VA. Deriving the pair from the rows themselves is what the row-scoped provider callbacks cannot do: GeoGenius.Provider hands normalize/2 and asserted_relations/2 one staged row at a time with no accumulator between them, and the row that needs the answer -- the ZIP whose state and county disagree -- is exactly the row that gets it wrong. The table is therefore stated here.

It is US knowledge, and it lives in the provider because that is what a provider is: GeoGenius.Providers.SimpleMaps parses one vendor's two US files and nothing else. Nothing outside lib/geo_genius/providers/ knows a FIPS code from a postcode.

What is in the table

The fifty states, the District of Columbia, and the five inhabited territories the Census assigns a state-level FIPS to: American Samoa (60), Guam (66), the Northern Mariana Islands (69), Puerto Rico (72), and the US Virgin Islands (78). Every one of them appears in uszips as a state_id and has counties or county equivalents under it.

Three codes deliberately absent are the Freely Associated States -- FM, PW and MH -- which FIPS 5-2 once numbered 64, 70 and 68 before the standard was withdrawn. They are sovereign countries the USPS serves, they have no US counties, and no row of either file carries a county_fips under them. Listing withdrawn codes would invite a five-digit number that happens to start with 64 to be read as a county of Micronesia.

AA, AE and AP are absent for a stronger reason: they are USPS constructs for military mail, not places, and no FIPS numbers them at all. A county can never belong to one, which is precisely the assertion the row's own state column was making before the prefix was consulted.