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.
There's a pretty easy fix:
take the layer that obscures the thing that you want to see and set it as an alpha-inverted track matte; basically, when the layer you want to see is covered by the alpha channel of the matte, it will be obscured. You get for free the fact that the matte goes automatically transparent 😉