Unity window button theme specification?

Unity window button theme specification?

I'm trying to track down the specification of Unity window button themes. By this I mean the specification specific to Unity, (/usr/share/themes/Radiance/unity), not the gtk or metacity specifications (/usr/share/themes/Radiance/metacity-1 or /usr/share/themes/Radiance/gtk-x.0).
The only themes I've found that appear to use this specification are the Radiance and Ambiance ones (in package light-themes).
Upon looking in /usr/share/themes/(Ambiance|Radiance)/unity, I see the images:
role.png
role_focusstate_state.png
where
role is one of close, maximize, minimize or unmaximize;
focusstate is focused or unfocused;
state is normal, prelight or pressed.
Is there some specification on how to create a unity window theme? Which of the above files is required at minimum? when each is used?
(For example, is the close.png ever used if close_[focusstate]_[state].png exists for all focus states and states already? If so, when? If not, why is close.png included with Ambiance/Radiance, which appear to cover all focus states and all states already?)