Rational Developer for System z
PL/I for Windows, Version 8.0, Programming Guide

Varying-length data sets

Your PL/I program can also create and access data sets where each record has a 2-byte prefix that specifies the number of bytes in the rest of the record. Unlike files with records delimited by CR - LF, these varying-length files can have records that possibly contain arbitrary bit patterns.

Rational Developer for System z
PL/I for Windows, Version 8.0, Programming Guide