If you reference a file into your current scene with shared display layers enabled, the display layers from the referenced scene are combined with those in the current scene (including those of any references). This avoids creating duplicate display layers when you want the same ones used throughout your scene, including the reference.
Maya uses the display layer name to determine how the referenced display layer is added to the current scene. If a display layer name already exists in the parent scene, any objects assigned to an identically-named display layer in a child scene are added to the original parent display layer when they are referenced.
If a child scene contains a display layer that does not exist in the parent scene, the display layer from the child scene appears in the Display Layer editor of the parent scene when the child scene is referenced. If these child scenes are later removed from the parent scene, their associated display layers are removed from the Display Layer editor as well.
Shared display layers are automatically placed in the default namespace.
All edits to referenced nodes that are shared are stored on one common shared reference node. Shared references can only be deleted from the Reference Editor using the Remove Reference menu item. The shared reference node remains for any references that were shared and is only removed when the last shared reference is removed.