The usage is more akin to :

x = getScriptForLanguage("aay"); // an undefined language code
or
x = getScriptForLanguage("$#!&("); // an ill-formed language code

// afterwards, x == "Zzzz" (unknown script)

That is, the API is returning a value indicating that what you asked for doesn't have a well-defined answer. 

On Sat, Dec 7, 2024 at 6:28 PM Guy Harris <gharris@sonic.net> wrote:
On Dec 7, 2024, at 5:24 PM, Mark Davis Ⓤ via tz <tz@iana.org> wrote:

> For internationalization identifiers in general, including languages, scripts, countries, and so on, we need some identifier value that means "the value for that identifier is unknown". It is also often used in APIs for "the value supplied was invalid.". Some examples are in https://cldr-smoke.unicode.org/spec/main/ldml/tr35.html#Unknown_or_Invalid_Identifiers.

So is there any specification for the behavior you'd get if, for example, LANG is set to "und_US"? Or is it "undefined behavior"?

> Around 2010, we added the "Etc/Unknown" in CLDR for use with the TZDB, to serve the purpose of a TZDB identifier for 'unknown'. The name was chosen so that it would be very unlikely to collide with any identifier that the TZDB would itself define in the future with a different meaning.

If there's any specification for the LANG=und_US behavior, what should an equivalent specification for TZ=Etc/Unknown say? If not, presumably the behavior is undefined, so making it equivalent to UTC, or McMurdo Sound, or whatever would be OK.