IntuitMike neglected to mention that you can have many additional custom fields, not just Other 1 and Other 2...

Open the Item List, edit any Item, and click on the Custom Fields button. Then you may define as many custom fields as your edition of QuickBooks allows. Once you've done that, return to the Invoice and customize it...you will see that the fields you defined in the Item List are now available to add as columns on the form. This gives you multiple blank custom columns.

However, you can also assign data to each/any custom field(s), Item by Item, in the Item List. Then, when you select an Item on a form, the data associated with it will be brought into the custom field (if it is included on the form).

You can also create additional custom fields for header of an Invoice, by defining them in the Customer List.