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

Macro precedence rules

When the same macro is defined in more than one place, the definition with the highest priority is used:

Priority
Definition
1 (Highest)
Command line
2
Description file
3
Environment variables
4
TOOLS.INI file
5 (Lowest)
Predefined macros (such as CC and AS)

If you invoke NMAKE with the Overriding Macro Definitions (/E) option, macros defined by environment variables take precedence over those defined in a description file.

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