Commission groups allow you to set different commissions for different users (affiliates) for the same campaign. One example of how this feature could be useful is for dividing your affiliates into groups. For example, affiliates from the VIP group will earn higher commissions than the other groups.
Post Affiliate Pro allows you to define an unlimited number of groups in every campaign and effectively separate your affiliates (e.g. by performance).
Using it together with the Performance Rewards feature, you can reward your VIP affiliates, or simply add performance criteria to your affiliate program.
For example you can pay 20% commissions if an affiliate delivers less than 5 sales per month, and 30% if he delivers 5 or more sales during a month. Any other time criteria that you desire can be set.