The new home for Visual Studio documentation is Visual Studio 2017 Documentation on docs.microsoft.com.
The latest version of this topic can be found at Macros and NMAKE.
Macros replace a particular string in the makefile with another string. Using macros, you can:
Create a makefile that can build different projects.
Specify options for commands.
Set environment variables.
You can define your own macros or use NMAKE's predefined macros.