I think it's a 3 letter code entering the US as well. The US uses 2 codes for visitors, as far as I know... at least those are the only 2 that I've received. One that directs you to secondary, and the other lets you pass. Both start with V for visitor.

As for Canada, I remember reading it all depends on the first number on the kiosk receipt. That first number will dictate whether you're selected for a secondary by the system. For entering Canada, if you're over your personal exemption, you get a $ sign printed on your kiosk receipt, meaning you'll have to go to the cashier or secondary to pay any extra duties and taxes.