VERIFIED SOLUTION i

How to find out where the MailStream Plus 'Shipping Services File' and Mail Piece File' write out to

MailStream Plus, all versions, all platforms
These files are selected via the IMBCTL parameter, position 28

Mail Piece File (X and M) 
On IBMi goes to work library into the M3XRFjobid file, M3XRF member 
On UNIX/Linux and Windows into the variable G1IMBXRF. 
On z/OS goes into DD G1IMBXRF 

Shipping Services File (S) 
IBMi- Variable format into MVSSFjobid file, MVSSF member. Fixed format into MFSSFjobid file, MFSSF member. 
UNIX/Linux and Windows go into the variable DD_MSNDSSF. 
z/OS goes into DD MSNDSSF
UPDATED:  August 10, 2017