What is the MailStream Plus MMBTAG (also called MSRDBTF) file

Product affected: MailStream Plus™, all platforms.
On the REPORT CONTTAGS parameter, position 52, there is an option to produce the MMBTAG/MSRDBTF file.  That file is a custom-format obsolete file that was useful back when Tray and Sack tags did not need to have IMb information on them.
It has not been kept up-to-date for several years, and there is a Note in the MailStream Plus™ (MSP) Guide in REPORT CONTTAGS position 52- 
"User-written programs should not use this file as input. Use Mail.dat files for all user-written applications.” 

The current version of MSP is 8.3.7.  In a future Release the option to generate this file will be removed.
UPDATED:  March 11, 2019