Pre-Assigned groups documented?

[Marc.Haber-lists@gmx.de (Marc Haber)]

Hi!

Is there a web page (or even a file locally on my Debian/GNU 2.0
system) that documents the purpose of the preassigned groups in my
/etc/group?

For example: There is a group called www-data. I might believe that
this group is the group of users that have write access to the web
server directory (/var/www on my system). However, that directory and
its content is owned by root.root.

Greetings
Marc

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