Enter a percentage (e.g. 5 for 5%). Every time a logged-in customer completes a checkout, that percentage of the order subtotal is credited to their digital wallet with the note Cashback #<order> - 5%. Leave it empty or 0 to disable cashback. You can override the rate for an individual customer on the Customers > edit customer screen in the Cashback % field (enter -1 to use the global default) — that field only appears when Allow Refer a Friend is set to Yes. The customer-facing Rewards page explains the programme and shows the current percentage when it is above zero.