What subkeyword is used indicate Periodicals mailing frequency in MailStream Plus?

MailStream Plus™, all platforms
The subkeyword MDMFFR is used, and you input the number of mailings per year in numeric form.  MailStream Plus™ will populate the "Issue Frequency" field on the 3541 Postage Statement, and the "Periodical Frequency" (CPT-1142) field in the CPT Mail.dat file.

(Note, prior to MSP Release 8.3.0, the subkeyword "ISSFRQ" could be used.)
UPDATED:  May 14, 2019