Check figure 1 at this site. That gives you an illustration of the lights. I added an explaination below.
http://www.uscgboating.org/SAFETY/fedreqs/equ_nav.htmYou can have a masthead light in combination with a stern light. Both are white lights. The masthead light is placed over the fore and aft centerline of the vessel and shows an unbroken white light over an arc of 225 degrees. The stern light is seen from behind so it shouw an arc of 135 degrees.
If you add up the arcs of both lights you will see that it covers 360 degrees, the same as one white 'all around' light.
The one white 'all around' light can be used instead of the masthead and stern lights on vessels under 12 meters (39.4 feet).
Figure 2 shows an all around white light.