Reboot

System → Reboot restarts the system or shuts it down. A reboot is rarely needed: settings take effect when you apply them, without a restart. You need it after a firmware update, or to clear a fault.

Active calls are interrupted in both cases.

Click Reboot. The page waits until the system is reachable again and then reloads by itself. This usually takes a minute or two; on a Raspberry Pi with an SD card it can take longer.

Shut down

Click Shut down before you remove the power, so the data partition is closed cleanly. The system can only be started again on site, by switching the power off and on (or from the virtualisation host).

In a container

In a container, the buttons act on the container:

LXC/Incus: Reboot restarts the container. Shut down stops it; start it again on the container host.
OCI container: both buttons do the same: they stop the container. Whether it starts again depends on its restart policy. With --restart unless-stopped (or restart: unless-stopped in Compose) it starts again by itself, so Reboot works as expected; to keep it stopped, use docker stop or podman stop on the host. Without a restart policy, start it again with docker start or podman start.