MPA3:Admin area/Settings/Two-Tier Rates
A two-tier affiliate program can reward affiliate for signing up sub-affiliates. As well as earning commissions on his sales, he can earn commissions from sub-affiliates sales, too.
Here's another way of looking at it: When you join an affiliate program, you are the first tier. The person you encourage to sign up is the second tier.
When adding a new two-tier pay rate you can set different two-tier rates for each affiliate.