Information MailStream Plus REPORT USPSSTMT and REPORT SUMMARY position 48 controls the Class of Mail they will print out for

In some mailing situations, it may appear that information is missing from the REPORT USPSSTMT or REPORT SUMMARY reports, or the report itself is empty.

The most common situation this occurs in is when you are presorting an unqualified USPS Marketing Mail.
The ML TYP parameter will have an S or 3 in position 8 indicating a USPS Marketing Mail mailing (formerly called Standard Mail.)
By default, if the mailing is below the USPS minimums, the pieces will be processed as single-piece rate First-Class.

If the job has a REPORT USPSSTMT to generate the Postage Statement, position 48 defaults to print for the Mail Class on the ML TYP parameter, which in this case is USPS Marketing Mail on a 3602 Postage Statement.  
But since the pieces went as First-Class and therefore go onto a 3600 First-Class Postage Statement, no Postage Statement will be generated.

The REPORT USPSSTMT parameter needs to have a 1 in position 48 to print the First-Class Postage Statement, where the pieces ended up.
In some cases there may be some qualified USPS Marketing Mail pieces and some unqualified pieces that moved to First-Class.  Then the job would need to have two REPORT USPSSTMT parameters, one to print the USPS Marketing Mail 3602, and one to print the First-Class 3600.

The REPORT SUMMARY parameter is similar.  This parameter generates a summary of the mailing, basically a summary of the Postage Statement and of the resources needed (postage amounts, and  number of trays/sacks/pallets, etc.) 
To get a full Summary report with all of the information in the situations above, you would need to have a 3 or S in position 48 of REPORT SUMMARY.  Or perhaps two REPORT SUMMARY parameters, one for the information from the 3602 and one for the information from the 3600.
UPDATED:  January 26, 2018