When in Delete Selection Mode, commands that insert text into the buffer first delete any selected text. Delete Selection Mode is a global editor setting. It is off by default with Emacs keys, and is on by default when using Mac OS X editor emulation.
Delete Selection Mode
Key Sequence: None
Toggles Delete Selection Mode, switching it on if it is currently off, and off if it is currently on.
LispWorks Editor User Guide (Macintosh version) - 9 Dec 2014