How to know whether to produce mail.dat or mail.xml files using MailStream Plus

Product Feature: Mail.dat / PostalOne!
Operating System: All Platforms

When using MailStream Plus™(MSP) should we produce mail.dat files or mail.xml files?
MSP does not produce the mail.xml files.  Using MSP you may only produces the mail.dat files when using REPORT MAILDAT to request the mail.dat files.
UPDATED:  April 2, 2018