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 running, 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.