full light - retains all texture and light information with shadows.
ambient light - is flat light with no shadows to enable the compositor to enhance base colours, or wash out the shadows.
occlusion - add depth and substance to the object.
the occlusion with the town does not work for some reason all we get is a stark bright white screen. after messing around with the amount of samples and max and min distance we establish it is nothing to do with the shader as the objects do not hold the characteristic jet black appearance in the orthographic viewports. so what to do......
we had decided that the scene had probably become corrupt due to the amount of shaders and imports involved with the scene.
we discussed the issue among us and still no resolution until clym came up with simply exporting all of the objects as a .obj then re-importing them into a new scene. this worked!!!!
by doing this we were able to get rid of all of the shaders and let maya concentrate on just the town and the occlusion.
so with this i sat down and spent the morning exporting and importing geometry chunks at a time in order not to crash maya, which happens frequently on my laptop. :D