Render layers: what is All Z, Zmask and Mask Layer Can anyone elaborate on what "All Z", "Zmask" and "Mask Layers" actually mean, giving some easy examples? I can see the pop-up explanations in Blender, but I am completely confused as to what it actually does (e.g. how it "masks out", what are solid z values, what in "fills in" the values of invisible meshes with and how it relates to masking, etc.).
When "Mask Layers" are used, any render information from the selected layer is omitted from the active render layer. 

In blender Z is generally used to determine depth and distance away from the camera. I can only assume "Zmask" would be omitting things base on distance depth away from camera. 

I do not have an answer for "All Z".