A number of the customers in Zuora are tax exempt. This information should be grabbed by AvaTax when it calculates tax.
The Certificate ID field on the customer record in Zuora is used by both the Avalara-built, and the Zuora-built integrations to pass the "Exemption Number" to AvaTax. As long as this field is populated with something, the customer will be exempted. This does also require that ECMS settings be turned on in the AvaTax account (to make any exemptions work at all).