> What does "sits in headings Canada taxes" mean?

My guess would be it's refering to the Harmonized System[1], used to classify goods on import and export declarations:

The HS is organized into 21 Sections, which are subdivided into 96 Chapters (Chapters 1 to 97 with Chapter 77 reserved for potential future use by the HS). The 96 HS Chapters are further subdivided into 1,228 headings and 5,612 subheadings in the current 2022 edition of the HS.

Most countries have additional subdivisions, for example in US the HS is used as a basis for the Harmonized Tariff Schedule[2].

The classification is used to determine tariffs, taxes and restrictions, but those are often applied to the broader groups, so covering say all imported aluminum, rather than say aluminum of a particular kind for use in a particular sector.

> Whatever this means, how does it jump to $26.5B from $16.0B on a single date?

Because Canada announced it will impose tariffs on more types of goods on that day, ie more headings, affecting more of the exported value.

[1]: https://en.wikipedia.org/wiki/Harmonized_System#Structure

[2]: https://hts.usitc.gov/