When you create the order, this field is copied from the and then used as a default value in the field. More information about how to create tax groups can be found in the Sales tax groups (form)dialog box.
The customer's tax group is transferred to the order and, from there, to the order line. Changes to the header field will only be updated in the order lines if set up to do this. See the button on the tab in the form. When the order is invoiced tax-related transactions are automatically generated. You can find additional information about how to set up tax groups in the Sales tax groups (form)and about how to set up tax codes and rates in the Sales tax codes (form).