What exactly do the three “special locales” called “en_US_POSIX”, “en_001” and “en_150” mean?

There are quite a few English-language locales on my computer system (Windows 10). Most of them look like this, and make sense: en_CA en_DE en_GB en_IL en_PK en_US … But there are three special ones, which don’t seem to correspond to a clear physical area: en_001 en_150 en_US_POSIX By doing some digging, I have extracted…

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