To create a new security rule, navigate to Documents > Settings > Security Rules and select Add.
This article will cover the security rule Referral URL:
Use the Referral URL security rule to restrict access to your document based on how the reader was referred it.
You can specify a number of web pages or domains that the reader should have come from when navigating to the document.
Typically, this option would be used to ensure that only visitors being referred from a company intranet site would be able to view the document. In this case something similar to intranet.mycompany.com/* would be entered.
To add a referral URL to the list, enter the URL into the New Referral URL field and select Add.
If you need to delete a Referral URL select it from the Current referral URLs field and click the Delete Selected button.