VERIFIED SOLUTION i

Learning how a post-MailStream Plus application knows what USPS Labeling List data was used for the presort?

Product affected: MailStream Plus™, all versions, all platforms
 
The Mail.dat CSM-1189 field is "Presort Labeling List Effective Date", and MailStream Plus™ populates it with the first day the labeling list being used is valid for mailing drop dates.  
For example, if using the Feb 2016 Postal Reference Files (PRFs), that field is populated with 20160301, since the first date they are valid for mailings dropping is March 1, 2016.
UPDATED:  May 15, 2019