JiminOhio is correct. If you fly LX to YUL, and then have a domestic connection in Canada, you will have to reclaim your checked luggage in YUL, take it through customs , and then drop it on the connecting baggage belt on the other side. Your bagage tags printed in Europe will say ZRH-YUL-YTZ, so you don't have to get a new tag in YUL. The process resembles that of intl-domestic connections in the US.

There is a trial in YYZ with AC which allows bags from certain inbound intl. flights to be transfered directly to domestic connections. AFAIK, this trial has not been extended to YUL yet. And it definitely hasn't been extended to LX.

If you need to declare goods, you will be sent into secondary inspection in YUL to do so. The LX flight arrives in a busy bank of European arrivals. I have waited as much as 45min in line at secondary around that time, but also as little as 5min when LX was amongst the first flights to land.