If Lifetime commission is enabled, whenever a customer makes a purchase referred by an affiliate, that customer would be automatically connected to the affiliate so the affiliate will get a lifetime commission for further purchases from the same customer.
To enable Lifetime Commission for a program, go to Program > locate the program you want > click edit (the blue button) > Advance > switch on Lifetime commission.
Once you have enabled Lifetime commission, you would have the option to set the first-time commission for the affiliate on the first order of any new customers referred by that affiliate.
By enabling First commission, your affiliate will get a different commission rule for the first commission while all future commissions will follow the program commission rate.
For example, if your Program commission is 10% and you set the First commission to be 20%.
If customer X buys from Affiliate A the first time, affiliate A will get 20% commission for that order, but for all other following orders bought by customer X, Affiliate A will only get 10% commission.