Payment reminder compared to dunning reminders are used whenever the risk process is shifted. In short this is used for all payment methods where the customer has to pay the merchant before the delivery starts.
The payment reminder feature allows you to automatically remind your customers to pay open invoices before the delivery starts. This will increase generally the conversion rate of payment methods like Prepayment, Multibanco, Boletto Bancario dramatically.
Payment reminders are activated under features and be found under payment > configuration > Reminders.