Mapplauncherd privileges question

Hello, how mapplauncherd uses privileges described in /usr/share/mapplauncherd/privileges.d/*.privileges?
Where “privileges characters” described?
As far as I understood from the source code, it starts the process from the “privileged” user in case there is any set of letters or a comma. Otherwise, it launches the process as the user who launches the application.

I’d like to understand these too. I was looking here: D-Bus APIs - SailfishOS Documentation

I’m trying to get screenshot ability again, which wored in 3.4 with simply adding:

echo /usr/bin/harbour-screenshot > /usr/share/mapplauncherd/privileges.d/harbour-screenshot

This no longer works. Ah, appending ‘,r’ works, but sadly kills my volla phone with 4.2.