MailStream Plus - "What specific Mail.dat files are required when uploading the file set to PostalOne! ?

MailStream Plus™, all versions, all platforms 
That's a question that cannot be answered except in terms of a specific mailing.  

Some Mail.dat files are always required, and MailStream Plus™ (MSP) always makes them. HDR, SEG, CSM, etc.

Some are not required unless you are doing a certain type of mailing. Like the .WSR is only required when you are doing Walk-Sequence mailings. If it's needed, MSP will then make that file automatically.

Some are required if you are doing a certain type of mailing, but you have to turn it on yourself on the REPORT MAILDAT parameter, like the CCR file.

And some files you have a choice of which you want to make.  Like (usually) if either the PDR or PBC file is needed for your mailing, the USPS PostalOne! rules allow you the choice of which one.

There are about about 20 files that may be required or optionally be produced by MSP.  MSP has DDs for all the ones that it might generate.  But usually a mailing job will only need to have around 8 to 12 Mail.dat files.

When you upload the files to PostalOne! the rule is that you only send them the files that have data in them, not empty files.
Note: on the Windows and UNIX/Linux platforms, MSP deletes any empty Mail.dat files at the end of the job, so any remaining files will be ones that have data in them.  But on z/OS (MVS), and IBMi, the user has to determine which have data and should be sent.

You can read more about the required/optional/choice status of the files in any Mail.dat Standards doc, located here-
You can determine the version of the Mail.dat files by looking in the REPORT MAILDAT report output after running an MSP job.  

There is also a USPS PostalOne! Technical Guide about the Mail.dat files, that can be found on the USPS  site here-
UPDATED:  May 14, 2019