Here are a few images to help you really understand the difference. The definitions that the Occlusion layer gives means that I can do a lot of Colour Correction without worrying about over-adjusting the shadows and creating washed out renders.
Those two are Occlusion layers of two separate buildings - (Left) General Store and (Right) Saloon.
In Photoshop I then load up the original render that was rendered in full texture, full colour and with HDRI lighting.
I then add another layer on top, paste over the Occlusion layer and then change the Blending mode to Overlay. The white goes and the black stays - meaning Shadows!
The definition is gives really, really makes the difference, and it's taking these models from works in progress to the finished article.
Next on the list is to build the camera rig and sooner rather than later we can start developing a little preview - and start getting some more followers.
Until next time, my few readers!
Jack Bromhead