L8437
Is this pressing "restart" from the power menu?
Any regular reboot triggered by the OS. Restart in the power menu, auto-reboot, idle reboot after update if enabled in the update client, reboot triggered by a device admin app, etc.
Am I guessing that a NON clean reboot is when the system crashes and reboots, but it takes alot shorter time to start?
You're referring to system_server crashing, which respawns system_server and other system processes. That's not really a reboot at all, although it's often called a soft reboot. It's not what is being referred to by an unclean reboot. An unclean reboot would be a hardware/firmware/software fault causing a hard reset without the OS cleanly shutting things down.