If you need to give your clients special tax rates or make them tax-exempt, you can adjust your store to meet this need by setting up tax rates specifically for different membership levels.
To set membership-specific tax rates:
Configure taxes in your store Admin area. See more details here.
X-Cart 5.4.x - Store Setup > Taxes
X-Cart 5.5.x - Store Management > Taxes
Disable the "Use the same tax rates for all user membership levels" option when configuring taxes.
Create a new tax rate or edit an existing one specifying the required membership.
Be sure to save your changes.
A detailed description of the tax configuration process is available in the section Taxes.