If you are looking at AC it is pretty easy to understand:

YVR-AC-YYC-AC-YWG-AC-YQT/YTO-
AC-CHI-AC-DEN-AC-BNA*

simply means you can do Vancouver - Calgary - Winnipeg - Toronto - Chicago - Denver - Nashville in that order. On the L fare there is no restriction on transfers so in theory you can go through all those places in each direction. However, that is not to say you will find flights that work, and note no stopovers (more than 4 hours connection time) are allowed (though you might be able to force an overnight if you arrrive somewhere on the last flight in and take the first fligth out the next day). Also, if you do it through AC reservations they cannot auto-price more than 8 segments and will need to send it to the rates desk in Winnipeg (Ask for Nicole!). You can get more than 8 segments to work on Destina if you pick your pairs of cities carefully. Expedia is less easy as you can only have 5 flights.
If you are thinking UA then the routing is more fun:

YVR-UA-YYC-UA-YEA-UA-YYC-UA-YWG-UA-YTO-UA-YOW-UA-PIT-UA-PHL-UA-WAS- UA-NYC-UA-BOS-UA-PIT/PHL-UA-CLT-UA-BNA*

OR

FROM YVR-UA-YYC-UA-YEA-UA-YYC-UA- YWG-UA-YTO-UA-BNA

Bottom routing is the same as AC. The first one though allows you to through all those new places as well. However, you are only allowed 4 connections in each direction, i.e. 10 segs total. You need to sit with a timetable and look for places with lots of connections and go from there

Hope this helps