Retention shows how many of the players you referred still play after some time. It is the other side of churn: if a third of players remain after six months, retention is one third and churn two thirds.
Why it matters to an affiliate
Under RevShare commission is calculated on the revenue players generate while they play, so retention, together with the rate, the revenue base and negative carryover, is one reason why two affiliates with the same number of FTDs receive different commission. Under CPA retention brings the affiliate nothing directly, but programmes track player quality and can lower the CPA for an affiliate whose players leave quickly.
How it is measured
Usually by groups of players by the month of their first deposit: what share of the group is active after the first, third and sixth month.