I think there is a side agreement limiting the number of Canadian car exports to the USA.
Need more info. But maybe the side agreement was in return for being exempted for any car tariffs that the USA may impose on “national security grounds”.
More details to come...
The car side agreement with Canada will probably be similar to the side agreement with Mexico:
“Mexico and the United States have agreed to a side letter to the new trade agreement announced Monday that will protect some Mexican auto exports against possible “national security” tariffs, Economy Secretary Ildefonso Guajardo said yesterday.
Under the arrangement, which Guajardo described as “insurance for Mexico,” if the United States government applies Section 232 tariffs to auto imports — as it did to steel and aluminum earlier this year — Mexico would still have duty-free access to the U.S. market for vehicles that comply with the new rules of origin, but only up to a certain limit.
Guajardo explained that the cap was set 40% above the 1.7 million cars Mexico currently exports annually to the United States, which he said gave Mexico’s auto industry scope to grow before facing the potential national security tariffs.”
source: https://mexiconewsdaily.com/news/some-exports-protected-against-national-security-tariffs/