Referrer Domains help you keep track of marketing effectiveness with any key referrer to your website. For example, if you have a partner named Smith & Co and their website is http://www.smithco.com, and you've got a partner page on their site, then it would be in your best interest to create a Referrer Domain for smithco.com. This way, any traffic that comes from Smith & Co's website would automatically get funneled into the Program/Campaign associated with your Referrer Domain. At any point in time you'll be able to click through the Referral Campaign to see how effective Smith & Co is at sending traffic your way.
What's even better, is you'll be able to measure the true ROI from the referring site. Suppose you spent $10,000 with Smith & Co. When a deal closes, we'll close the loop and tie that closed-won deal back to the fact that your new customer came from Smith & Co. If you close two deals worth $20,000 total and they came from your $10,000 Referrer Campaign with Smith & Co, you've just netted 100% ROI on your marketing with Smith & Co. Nice job!
First, make sure you have a Program to put your Referrer Campaigns into. We suggest using "Referrers". You could also separate it out, and have a Program called "Partners", and one for "Channel", etc. To create a new Program go to Marketing > Programs > New. Next, create a Campaign. We suggest using the name of your channel or partner for the Campaign. Make sure to set a cost for your Campaign. Learn more about Programs and Campaigns here.
Finally, setup a Referrer Domain navigate to Settings > App Setup > Referrer Domains > New.