MailStream Plus- How to use Fixed-Length or Variable Length Name-Address records on the z/OS platform

Product affected: MailStream Plus™, all versions.  On the z/OS platform
The FILEDF parameters are used to describe input and output files. Position 17 is used to indicate whether the records are Fixed-Length or Variable length.  That tells the Input/Output program MailStream Plus (MSP) uses how to read and write the records.

On z/OS an 'RDW' is the 'Record Descriptor Word'.  It's the first 4 bytes on a *Variable Length* record on the z/OS platform.  The actual data area starts after that, in position 5.

Fixed Length records
Fixed Length records do not have an RDW.  They are all the same size, and the data area starts at position 1.
So for example in a fixed length record if the ZIP Code started in position 1, then the ZIP IN parameter would look like this-
ZIP IN 001 C

Variable Length Records
The MSP basecode itself does not know whether the record is Fixed or Variable length.  For internal reasons, the entire record is input into MSP for use, and for Variable Length records that includes the RDW.  So the 4-byte RDW length must be included in all positions specified on parameters.  For Variable Length records where the ZIP IN begins in position 1 of the data area, the parameter would look like this-
ZIP IN 005 C
That means that counting starting at the first byte in the record (remembering that the 4-byte RDW is included in the record that MSP sees), the ZIP Code starts right after the RDW. 

For positions specified on Variable Length records, the same applies to all input and output record locations in the job.  The 4-byte RDW must be accounted for.
UPDATED:  July 11, 2019