Expose Tax Registration Number of Customer on Invoices

1. Navigate to Setup > Customization >  Transaction Body Fields >New

2. Set the following:

Label = Tax Registration Number (or any appropriate name).

Type = Free-Form Text

Store Value = T

3. Under Applies To subtab, mark Sale checkbox.

4. Click on Sourcing and Filtering subtab and set the following:

Source List = Customer

Source From = Tax Reg. Number

5. Hit Save.