man systemd.special states it is:
A special service unit for shutting down the system or user service manager. It is equivalent to poweroff.target on non-container systems, and also works in containers.
systemd will start this unit when it receives the SIGTERM or SIGINT signal when running as user service daemon.
Normally, this (indirectly) pulls in shutdown.target, which in turn should be conflicted by all units that want to be scheduled for shutdown when the service manager starts to exit.
My understanding is that it is for shutting down the instance of systemd (whether user or root) that is asked to target it. However, the term "non-container systems" confuses me. Aren't all modern Linux kernels effectively "container systems"? Is this here so that the man page still makes sense if someone tries to port systemd to some other Unix? Or does this refer to some characteristic of systemd-based Linux distros I am unaware of?