Python .pth and ._pth files (standard install vs. embedded Python)

Question

I see two diffrent behaviours about .pth and ._pth files:

• the official Python install for Windows uses .pth files in lib\site-packages as documented in https://docs.python.org/3.12/library/site.html#module-site. It seems that adding multiple .pth files works and is used at the Python interpreter startup.

• the Python Embedded package for Windows seems to use ._pth files (and not .pth!) for the same thing: python38._pth, but in the same folder as python.exe. In this case it seems that adding a second .pth or ._pth file in the same folder isn't used by the interpreter. Why? Is it documented that, for embedded Python, only one ._pth will be used?

More generally, what are the rules about .pth vs ._pth files? In which case should we use one or the other, what is the reason for the underscore _pth?

Note: the rules seem here complicated (.pth vs ._pth ; multiple files allowed vs. only one file used).

NB: Documentation about ._pth files