Is stick out factor a fraction of radius or diameter?

I got confused by this sentence in tree.py:

    .. attribute:: stick_out_factor

        The fraction of the box diameter by which the :math:`l^\infty` circles
        given by :attr:`source_radii` may stick out the box in which they are
        contained. A scalar.

I think the code treats it as a fraction of the box radius? I am not 100% sure. The error estimates have been treating it as a radius.