Yesterday flying Air Canada Jazz I checked in 2 hours ahead of time. There was no seat on my boarding card, the agent said my seat would be assigned at the gate.

At the gate after some wait I was assigned a seat shortly before boarding. The gate agent told me "we always overbook flights" and that was why I had no seat assigned. If this happens again, and if the flight is overbooked, what will happen? Will I automatically be the one who does not fly if I have no assigned seat. Or will they go through the "offer people flight vouchers" thing to decide who gets bumped.

I assumed that a confirmed ticket meant I would fly that day, it looks like I can't be sure of that, I want to know my rights for next time.