Support for setting up Affiliate plugin


I just need a little clarification on how the affiliate plugin works. I want to set it up so my current paid users can refer others to sign up for a paid site. I am using Pro Sites and in the Pro Site Settings I see I can set payment amounts for each level.

So, for example, say the monthly price for my service is $20/month. If I set the "Affiliate payment for one month" to $5, does that credit the current users PayPal account $5, or does it discount their charge on the next payment to $15? Also, does the $5 only occur once, or is it a discount every month for them?

Thanks, Chris