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.