I have never seen the Canadian code. It says to me that every drain pipe must be vented, from the moment of installation.

seems to be saying that one must connect venting at time of installation. If you leave it until tomorrow, you might not be back the next day, for an infinite number of good reasons, but good intentions don't count. One may not make the claim to have left it without venting because one intended to be adding venting "soon". This is how I read it.

I agree that is what it sounds like, but they don't make a point of saying that about anything else. It would not pass without the vent. Why don't they say that a "trap" must be put on at the same time? I just don't understand why they singled out the "back vent". I appreciate your responses.

venting is needed for the system to work, as a system.

Any trap arm can be temporarily disabled; you just block the pipe by putting a sock in the pipe. With one trap arm inoperative, no harm is done. The vents are all still required, as a system requirement, because the other trap arms may be operational already.

venting makes drains work as planned. So it is as important as the drain pipe itself.