NUT offdelay question

[MisterE2002]

I am following the great NUT guide at http://rogerprice.org/NUT/ConfigExamples.A5.pdf
But this part is a bit unclear to me.

9. upsmon calls the SHUTDOWNCMD specified on line 45.
10. We now enter the scenario described in figure 15. The operating system's shutdown process takes over. During the system shutdown, the Bash script shown in figure 16 or equivalent systemd service unit or some other equivalent runs the command upsdrvctl shutdown . This tells the UPS that it is to shut down 20 seconds later.

So, is "offdelay" the complete time for the OS to shutdown (default 20s seems impossible for TrueNAS).
Or...
The OS will shutdown and AT THE END it will sent something to the UPS to inform it to shutdown in 20s.

Even in this last case. How to measure this? I assume this call is not logged.