What it is:
Box 13 is where the signature indicating authorization of payment for medical benefits to the provider of service. This can be done by entering "Signature on File", "SOF" or the actual signature.
Therabill will put the text "Signature On File" into box 13 if the Signature on File box is checked for the insurance card being used for the claim being billed.
On the top menu bar, select Clients / Client List. Click the blue pencil for the desired client. Click Add Card or the blue pencil of an existing insurance card. You will now see the Edit Insurance Card form. In the Advance Information section find the Signature on File: checkbox and the Information Release: dropdown.