In a camera tracked environment, sometimes you want to have objects hidden by a wall or something.
In 3D software you achieve this be placing some geometry in the scene the mimics the wall in 3D space, obfuscating other objects, but is itself transparent by way of special material or compositing tags.
How can this be achieved in after effects?
In 2D I would just create a duplicate of the layer, create a mask for it, and layer on top. But this is a 3D camera tracked scene so doing this would require a complicated mess of nested pre comps and time consuming rotoscoping of masks.
The "Accepts Shadows:Only" is a similar idea in that it shows shadows, but is itself invisible.