NMAKE inherits all current environment variables as macros. For example, if you have a PATH environment variable defined as PATH = C:\TOOLS\BIN, the string C:\TOOLS\BIN is substituted when you use PATH in the description file.
You can redefine inherited macros by including a line such as the example above in a description file. While NMAKE is executing, the macro takes on the redefined definition. When NMAKE terminates, however, the environment variable resumes its original value.
The Override Environment Variables (/E) option disables inherited macro redefinition. If you use this option, NMAKE ignores any attempt to redefine an inherited macro.
The macro name, for any macros that you define, is case sensitive. For example, consider this macro:
UPPER=UpperCase
In this example, $(UPPER) returns the value, but $(upper) does not. Inherited macro names (i.e. those created automatically from environment variables) must always be UPPERCASE.