Article 2 of the treaties deals with what taxes will be covered under the treaty. In Canada, this usually refers only to the Income Tax Act. So that means that GST, excise tax and provincial taxes are not covered.
What taxes are covered in the other jurisdiction depend on the wording of this article. For instance in for Argentina, the taxes covered under the treaty include the income tax, the assets tax, and the personal assets tax.
Many treaties also include a paragraph in this article to include taxes imposed in each country that are identical or substantially similar if they are created after the date of the treaty. This allows countries to change legislation without immediately having to go back and negotiate treaties.