Navigate to: System Settings > Global Settings > Promotions and Coupon Settings Tab

Fields | Description |
Allow Promotion And Coupons Together | When this field is enabled admin can apply promotions and coupons together or individually to order. When this field is not enabled promotions or coupons can only be applied individually to order. Note - By default, this field is checked as Yes |
Promotions With Exceptions | Enter Comma separated Promotion Classes for Promotion that can be used along with Coupons even when Allow Promotion And Coupons Together is disabled. |
Calculate Tax On The Discounted Price | When this field is enabled, the tax is calculated based on the discounted price of the order. E.g. If the order total is $20 and the discounted price is $17, the tax will be calculated at $17. |