Some jurisdictions (such as certain Canadian provinces) require that you charge multiple Tax rates on orders. For example, you may need to charge an HST tax along with a GST and a PST on your store orders.
When adding Sales Tax calculation to your store, you will see the following popup modal where you will select the country that the taxes will apply to.
When selecting Canada (or any country other than the United States), the modal will give you three separate fields for your individual tax rates.
You may also select the "Include Previous" checkbox for Tax 2 and Tax 3 if you need to calculate the additional tax based on the previous rate charge.