For sdist, egg and wheel release from : https://setuptools.readthedocs.io/en/latest/userguide/dependency_management.html#platform-specific-dependencies
Sometimes a project might require a dependency to run on a specific platform. This could to a package that back ports a module so that it can be used in older python versions. Or it could be a package that is required to run on a specific operating system. This will allow a project to work on multiple different platforms without installing dependencies that are not required for a platform that is installing the project.
setup(
name="Project",
...
install_requires=[
'enum34 ; python_version<"3.4"',
'pywin32 >= 1.0 ; platform_system=="Windows"'
]
)