All states have county code listings using 3 digit codes, but if a facility needs to add an out of state county code, it isn't possible if the 3 digit code already exists in that state. The federal government Bureau of Labor Statistics and the Census Bureau use FIPS (Federal Information Processing Standards) codes for each state along with the County code. These are ANSI recognized codes that can be found at https://www.census.gov/library/reference/code-lists/ansi/ansi-codes-for-states.html
The State-County Code list can be found at
https://www2.census.gov/programs-surveys/decennial/2010/partners/pdf/FIPS_StateCounty_Code.pdf
Using the FIPS code along with the county code creates a 5 digit number that's unique and would enable facilities to add any county within the US and it's territories.