Who understands the repercussions of all these tariffs?

The US tariffs on Canadian exports are hard to understand - both the theory and the practice.

For example, I read that "Canadian aluminum, steel products dealt biggest blow from US duties". But isn't aluminum and steel covered by CUSMA, and therefore exempt from the tariffs? Is it, then, just a small percentage of aluminum and steel exports that is actually being tariffed? Aluminum and steel are being at a different rate (50%), and with a different "justification" - does this invalidate the CUSMA defence? None of this clear to me.

I also read that between 80% and 90% of Canadian exports to the US are covered by The CUSMA agreement, and therefore exempt from the 35% tariffs we are supposedly subject to (other than energy and potash sales, which attract a 10% tariff). I have seen figures of 86%, 90%, 95% - there does not seem to be a definitive figure. 

But that seems to be a theoretical figure anyway, and many businesses have never bothered with the onerous paperwork of proving that their products are CUSMA-compliant. So, the actual figure is probably a lot less, but no-one seems to really know. Many smaller companies that have never officially registered their products under CUSMA are now scrambling to do so, so the actual figure is probably something of a moving target.

I don't know whether Trump understands all this - maybe he's smarter than he seems - certainly he has a whole department keeping track of it all for him. But I have a suspicion that hardly anybody really understands it all in detail. The Canadian case may be more complicated than most, what with the CUSMA to take into account too. Presumably, somebody in the Canadian civil service understands it pretty well, and is keeping track of it. Me, I've just about given up.