Here you'll find global settings for the creation of Salesforce accounts and Zuora billing accounts. First, specify the bill cycle day (default is 0 for "Auto-Set") and whether Country Codes are enabled in Salesforce. This setting must be set correctly in order for the check-out process to function correctly.
By default, customers are tax exempt if they specify a VAT Id. You can either exclude certain countries from following this logic, so that they will not become tax exempt, or you can explicitly whitelist certain countries, so that customers from other countries will not become tax exempt, regardless of entering a VAT Id or not.