The Tax Number field displays the Contact's Tax Identification (such as Seller's Permit or Sales Tax Exemption number) for Contacts that do not pay sales tax when buying taxable Inventory items.
This field is for reference only and can be helpful (when completed) if the store is audited. It is displayed on Sales Transactions and Sales Tax Reports. It is not integrated into any other program functions and may be used for other purposes without complications.
See:
Contact Database - Main Screen Fields