There's a nice explanation stating there is something like "group that the processes are executed as". However, I could not find what this exactly is. I noticed that setting the "Group" option in the systemd script changes permission on where files can be created. That is, without the option I was allowed to write a certain file, but with it I was not.
This suggests that the "group that the processes are executed as" does more than just determine what the group of newly created files are, it also elevates or reduces permissions.
Unfortunately, there is also something called a "process group" which is like a set of processes, which would not change permissions and rights I would think. But this makes the feature harder to google.
Could someone explain to me what the "group that the processes are executed as" is?