Hello,
An IRO would allow a second domain name to show the same content as the existing domain.
The address in the Browser location bar would reflect the domain that your visitor typed in and would not change unless you implemented a .htaccess redirect or your internal links referred to the different domain name.
You would be able to have unique email addresses for each domain, as noted here:
https://FutureQuest.net/forums/showthread.php?t=27415
-Bob