This is a userspace component — you cannot attach it anywhere but the userspace. Messing with the userspace can be fun, but it is not recommended as you risk messing your dash up if you don't know what you're doing.
The OverlayLayer component instructs the renderer to render the hierarchy it is attached to on top of everything else. In desktop mode the component also causes the hierarchy to render directly in front of the camera, mimicking a 2D user interface.
Controls whether or not this component is enabled. Some components stop their functionality when this field is disabled, but some don't.
Behavior
When enabled, OverlayLayer will tell the renderer to render the hierarchy under the slot (including the slot) on top of everything else. Internally, this is done by setting the layer of each GameObject in the hierarchy to Overlay.
Examples
OverlayLayer is used to display the dash, notification panel, and the cursor on top of everything else in userspace.