What is a referring domain?
Some definitions to get everyone on the same page:
Referring domain: also known as a ref domain, a referring domain is the domain that backlinks are coming from.
Backlink: a backlink is a link on another website that points to your site.
When should I use this feature?
This feature can be useful to:
Apply a promotion only when the traffic comes from an affiliate
Restrict a promotion to traffic coming from a social network
Exclude traffic coming from search engines
...
How do I use it?
Edit your promotion
Go to the "Rules" tab
Find the Referrer section
Choose to include or include domains and add your domains, separated with a comma: