Scores - Note Layer
If you are moving and editing notes, you might accidentally move other objects nearby. To avoid this, you can assign different types of objects to different Note Layers (up to 3) and instruct Cubase to lock one or 2 of these layers, making them unmovable.
This page is where you specify to which layer each object type belongs. The actual locking of layers is done on the extended toolbar of the Score Editor.