Comment 16 for bug 1670291

/sbin/shutdown, post upgrade not working, imho is a severe bug. I'll investigate that. I suspect that because we cannot restart dbus things don't show up there, but /sbin/shutdown should be smart enough to do something. I guess the scheduling of timeout is failing, but /sbin/shutdown now works?

Re differences between halt & poweroff, in systemd, eventually it results in:

case ACTION_HALT:
    (void) reboot(RB_HALT_SYSTEM);

case ACTION_POWEROFF:
    (void) reboot(RB_POWER_OFF);

For completeness reboot typically is:
(void) reboot(RB_AUTOBOOT);

of if parameter was specified (aka androidish - reboot recovery):
(void) raw_reboot(LINUX_REBOOT_CMD_RESTART2, parameter);

This is the same as with upstart's implementation:

case REBOOT:
    reboot (RB_AUTOBOOT);

case HALT:
    reboot (RB_HALT_SYSTEM);

case POWEROFF:
    reboot (RB_POWER_OFF);

case REBOOTCOMMAND:
    syscall (SYS_reboot,
       LINUX_REBOOT_MAGIC1,
       LINUX_REBOOT_MAGIC2,
       LINUX_REBOOT_CMD_RESTART2,
       rebootcommand);

Differences between RB_HALT_SYSTEM and RB_POWER_OFF can be read in the linux manpage here: http://manpages.ubuntu.com/manpages/bionic/en/man2/reboot.2.html

I have not read kernel code in a while for this, but if i'm not mistaken poweroff fallsback to halt, in the kernel, if there is no hardware support to kill power.