The API of Mail & Deploy is an HTTP web service that allows 3rd party vendors to directly access certain functions of Mail & Deploy. Using the API, Mail & Deploy reports (and thus QlikView and Qlik Sense) can be strongly integrated into existing applications.
Example: If you are the developer of a CRM system, you can easily develop interaction between the CRM system and Mail & Deploy; if, for instance, you have customer reports in Mail & Deploy that contain data from QlikView and/or Qlik Sense and you for a given Customer ID and the API will return the report document file to the caller.