Got it....Thanks.
Thanks, KVS.
Got the end-on-end. Don't know why I didn't find that site in the google search I did.
The link didn't have a definition for "add-on" but I now realize that the "add-on" is the "proportional fare" for the leg to the gateway fare (ex: SFO-ORD for the gateway fare ORD-LHR).