The codes also tell quite a lot about the immigration officer's discretion. Sometimes when you approach them, they will assess where you came from by asking you where you originated your journey and the last time you left canada. All to see if you're condricting the info you present on your customs form. They will sometimes ask for your itenitary even if its a E-ticket.

Last year I had to go through secondary when arriving YVR from LAS, which was the first time I've done that routing (drove into YVR and flew to LAS and back). Apparently the routing you do can trip flags -- I'm pretty sure they communicate with the US immigration folks upstairs at YVR.

Sanosuke!