What are the secondary instances of systemd for?
When systemd runs as PID 1, it is just a replacement for init. It creates userspace and subsequently acts as a babysitter for daemons. That is well-documented.
But it is also possible to launch additional instances of systemd, running either for the system or for a user. In fact the pam_systemd.so module automatically creates such an instance for each new user who logs in. What exactly are these for? What, if anything, do they do? I can't find any documentation on them at all.
No flames please! I know systemd is controversial but I am neither for nor against it. I built my current LFS with systemd simply because I wanted to do something new and learn something.