The tax system in Kartris works differently for US/simple models, and EU tax.

The tax rates page itself is only really active when the tax regime is set to EU. It will list various rates which can be used for the various bands of VAT in your country. For example, in the UK these would be 0%, 20%, 5%, etc.

Once these are set, you can choose which tax band applies when creating or editing product versions. For example, in the UK some items such as childrens' clothes are exempt from VAT, while most items will come under the 20% tax band.