As it’s been the subject of several emails to support, and it wasn’t 200% clear to me personally until earlier this evening, I’ve written up a new documentation page about dealing with taxes in SimpleCart. It explains the different levels of tax rates that can be at play, and how these levels fallback on each other.
Hopefully the documentation clarifies it a bit, but be sure to post here (or in a new topic) if you have any further questions. We’ll continue clarifying the documentation where needed.
(The 2.3.0-rc7 release the documentation talks about will be released coming week)