Q:

What is layer exclude in blender?

Hi guys i was wondering if you can explain me what is exclude layer and what it does ?. I watched tutorial about layers it was about changing scenes visible and invisible and almost couldnt understand 

A:

Scene layers (all objects that influence the render, directly or indirectly) are
shared between all render layers. However sometimes it's useful to leave out some
object influence for a particular render layer. That's what this option allows you
to do.
For a layer to have any role in the scene (not necessarily in render output or interactions with light), it should be part of the 'scene' layers. For example, masking.
In this example, the cube and lights are in layer 1 while the sphere is in layer 2.

Lets say we want to use the cube as a mask but don't want it to influence the other aspects of the scene (say shadows from it). Then we can do something like shown below.

We don't want the sphere to be rendered, so layer 2 is unchecked from 'layer' layers.
We also don't want the shadows of the sphere to fall on the cube, so we excluded its influence by checking it in 'Exclude' layers. But we want the sphere to be used as a mask, so we checked it in 'Mask' layers. But had layer 2 not been present in the 'Scene' layers, even if we checked it in 'mask' layers it wouldn't have any effect as shown below.

Another purpose of 'Exclude' layers is for better organization and readability of render layers in some cases. 

