The documentation for Mathematica needs to include a set of (static or dynamic) dictionaries at least showing usage, if not explaining the behavior for several constants used in WDF, e.g. "AllUSStatesPlusDC". Does this already exist?

I can create a dynamic list of properties for ChemicalData[] using ChemicalData["Properties"]. But I cannot find a page (or a method to create) a list of counties in the US.

The closest I have found so far is the Related Interpreters section of the USCounty page in the help system (ref/interpreter/USCounty). I could not figure out how to extract this using WolframLanguageData[], no luck with

WolframLanguageData["USCounty", "RelatedInterpreters"]

More of the same problem!

Mathematica help provides no reference to these strings. Google finds no Wolfram-supplied Mathematica-related reference to these strings.

Thanks in advance for any suggestions.

Motivation

The suggested answers to the problem described here at this link

included the following constants

unknownConstants = {"ParentRegion", "AllUSStatesPlusDC", 
  "ContinentalUSStates"}

And also these bizarre strings concatenated with state names (e.g. "USCountiesIdaho"):

stateNames = #[[2, 1]] & /@ 
   EntityList[
    EntityClass["AdministrativeDivision", "AllUSStatesPlusDC"]];
unknownConstants = 
 Join[unknownConstants, ("USCounties" <> #) & /@ stateNames]

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