Precedence in Macro Definitions
The latest version of this topic can be found at Precedence in Macro Definitions.
If a macro has multiple definitions, NMAKE uses the highest-precedence definition. The following list shows the order of precedence, from highest to lowest:
A macro defined on the command line
A macro defined in a makefile or include file
An inherited environment-variable macro
A macro defined in the Tools.ini file
Use /E to cause macros inherited from environment variables to override makefile macros with the same name. Use !UNDEF to override a command line.