command line usage, -P flag

[vanderaalle]

I'm calling musescore from python via terminal and I am able to render a score to pdf via -o option, referring to this page:

https://musescore.org/en/handbook/4/command-line-usage

This is an example of a call I generate:

call = "'/home/andrea/Applications/MuseScore-Studio-4.5.2.251141401-x86_64.AppImage' -o '/home/andrea/test.pdf' '/home/andrea/test.mscz'"
print(call)
os.system(call)

Fine. Instead, I'm having hard times understanding the -P flag. E.g.
call = "'/home/andrea/Applications/MuseScore-Studio-4.5.2.251141401-x86_64.AppImage' -o '/home/andrea/test.pdf' -P '/home/andrea/test.mscz'"
print(call)
os.system(call)

First, it's not clear how it works. Secondly, from my tests:
i) I have to generate the parts in the file in order to have them rendered (but documentation say it would work even without doing it, am I right?)
ii) even if I do as in i), the only thing I'm getting is a single pdf with the last part

Am I missing something, or is it a bug?
Thanks