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

Creating DLL source files

To build a DLL, you must first create source files containing the data or routines that you want to include in your DLL. No special file extension is required for DLL source files.

Each routine that you want to export from the DLL (that is, a routine that you plan to call from other executable modules or DLLs) must be an external routine, either by default or by being qualified with the external keyword.

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