You should not use PDSEs if your application processes SAM null record segments. Null record segments, which are created only with variable blocked spanned (VBS) records, are not saved when the data set is written. For example, if you create a null record segment when writing the data set, it is not returned when the data set is read. On the first write of a null record segment, a message is written to the job log and an indicator is set in the SMF record type 15.

Related reading: See z/OS MVS System Management Facilities (SMF) for more information about SMF.