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

IBM.SYSLIB

The IBM.SYSLIB environment variable specifies the primary input directory search path for include files identified by %INCLUDE statements in your source program. For example:

  set ibm.syslib=c:\pli\project\updates;\pli\system

These directories are searched before any directories specified in the INCLUDE environment variable.

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