The notification APIs needs to be implemented on the Initiating Party side. The TPP solution will post notifications to these endpoints. It can, for example, sent a notification when the status of a payment has changed. 

For the iDEAL product the post status notification is part of the product, a value-added service is not required (because the notification is part of the iDEAL scheme and the TPP solution doesn't have to-do additional polling).

For other products the notifications are a value added service.

Below an example of a notification in a redirect flow.