What goes into setup.py's 'requires' field?

edit

The standard setup.pyfor a ros package looks something like this:

from distutils.core import setup
from catkin_pkg.python_setup import generate_distutils_setup
d = generate_distutils_setup(
    packages=['package_name'],
    package_dir={'': 'src'},
    requires={'rospy'}
)
setup(**d)

I am a little confused as to what goes into the requires={} field. Is it a list of python package requirements similar to what is in package.xml, or something else?

For what it's worth, most code in github doesn't seem to have examples of this being used.

In particular, if my code depends on scikit-learn, what should go in the requires field?