If you charge sales tax, you might have some customers that are exempt from paying sales tax. A setting in each customer's record helps you keep track of this.
To mark a customer as tax exempt:
1. Go to Customers and click on the customer, OR Open a booking and go to the Customer tab.
2. In the Tax exempt section, from the Taxes  scroll down menu, select if the customer
is exempt from all taxes
the customer is exempt from the following taxes, and tick the Taxes  from which the customer is exempt
the customer is not exempt from any tax (this is the default value)
If you mark an existing customer as tax-exempt, this will change the price of all the existing bookings for that customer, including past bookings.
A negative balance could show for past bookings if they had been fully paid.