ATA Security User: user or master?

[glauco]

Hi,
I'm figuring out how to enable hardware encryption on my self-encrypting drives (SEDs).
I'm at the System > Advanced page of my TrueNAS-12.0-U1 (latest) web UI.
Does anybody known when I should select user|master from the ATA Security User dropdown menu?
The blue info thingy says that that value is passed as an argument to command camcontrol security -u.
I read the fine man page but it didn't enlighten me.
What puzzles me is that camcontrol defaults to master, whereas the TrueNAS web UI defaults to user.
This truenas.com/docs page on SEDs just says "Go to System > Advanced > SED Password and enter the password." No mention of ATA Security User.
Thanks to the brave one who'll shed light!