"LaRCS" stands for "Live and Record Combined Stereo", which means that on the left you can hear a live performance, while on the right the clip version.
Regarding the current video, you can enjoy the title duet of "The Phantom Of The Opera" from "The Phantom Of The Opera" (from Andrew Lloyd Webber), by Sarah Brightman: on the right side the official promo video clip together with Steve Harley (tempo adjusted), while on the left side a live version with Antonio Banderas from 1998 (the Royal Albert Hall, gala celebration of Andrew Lloyd Webber's 50th birthday). Links:
http://www.youtube.com/watch?v=MwialGRHXqU
http://www.youtube.com/watch?v=x1BgXhFv-ao
http://www.youtube.com/watch?v=Ny5H9GiVP_0