We have a field called Business Number where you can put your Australian Business Number or your Tax ID. This is attached to the basic information for a specific personality, and if it has a value, will appear under your Business Name on your printed Estimate, Advance Invoice or Invoice documents.
If you click on your name in the top right corner < My Accounts < Personalities, and choose a specific personality, you will be able to enter this Business Number under the Basic info tab.
You can also have a Business Number attached to a specific contact. In the address book when entering basic information for a contact, you can add a Business Number (ABN or Tax ID) to appear with the contact’s information on the document.