TrainerCentral allows you to collect the course fees in installments. For instance, consider a course called 'Business English' that is offered for $50. The admin has the option to charge $10 for 5 cycles.
The main difference between a subscription and a payment plan is that with a subscription, customers are charged until the course or ticket is canceled, while with a payment plan, customers are charged based on the number of payment cycles mentioned by you during the ticket configuration.