The Tax Table field displays the Contact's Tax Table which is used to define the tax rate the Contact pays when buying taxable Inventory items.
A Tax Table does not need to be assigned to each Contact. If the field displays the NULL characters, the Tax Table on the Customer record is used.
If the Contact is charged a different Tax Rate than the Customer Record to which the Contact belongs, navigate to the Tax Table field and press [Ins] to select the correct Tax Table for the Contact.
See:
Contact Database - Main Screen Fields