This is optional feature of Post Affiliate Pro, which gives you option to have more users with role merchant in same affiliate program. For each of your merchants you can define role with rights on different application parts.
e.g. if you like to have merchant user, which has right to communicate with your affiliates, but not define campaigns, you can define special role and assign to this merchant user role you defined for this type of affiliate.
!!! IMPORTANT !!! - This feature doesn't mean, that Post Affiliate Pro can be used as Affiliate Network software! Affiliate network software separates merchant accounts, but this feature allows you to have more merchant accounts with different rights to see same data.