It is normally used for both unlock and login.
It is a generic
implementation of a Secure Attention Key (SAK),
without which Linux is
actually less secure than Windows.
Never heard of it, nor have I ever seen CTRL+ALT+Del in Linux
For years I have been using Ctrl-Alt-Del to exit KDE. I also for many years have been using Ctrl-Alt-Del at the console to reboot.
Lots of people disable the latter for true multi-user systems.
The SAK support can be turned off in the KDM control
center module.
I don't see it anywhere under Login Manager or KDM Theme Manager. These are the only two modules that show up when I type kdm into the search box for kcontrol.
Under the Appearance tab, last item on the bottom. If you don't have that option was your package built with SAK support?
Darrell