Thank you number_6. I think I understand now.

I just want to check with you my understanding though:

Assuming I can get my onward boarding card and through-checking of my bags at LHR (which I think I will be able to as I'm on an AGLOB34) I do collect my baggs, pass through immigration and customs, but I then drop my bags off at the transfer desk and then go back airside using my onward boarding card?

I assume all this collecting and re-checking of baggage has something to do with the "first port of entry rule" that the USA has. I know Canada isn't part of the USA, which is one of the reasons I routed through YVR, but I guess they have some kind of "arrangement" with the USA. I'm trying to remember when/if I ever did an international to domestic transfer anywhere else but I can't. The only ones I remember are international to international and I through-checked all of those in the conventional sense. None involved the USA or Canada obviously. I'm trying to figure out if I have just been lucky with my routes and airlines or if this is just a USA/Canada/maybe one or two others thing.

Thanks.