CMS Desk > E-commerce > Configuration tab > Departments > Edit department:
On this tab, you can choose one of the tax classes shown in the list. The selected tax class will be applied to the products in the currently edited department. You can also edit the tax classes in the list by clicking the appropriate Edit () icon.
Help URL: http://devnet.kentico.com/docs/6_0/contexthelp/index.html?cms_ecommerce_configuration_departments_taxclasses.htm