Currently there's a limit of 5 layers.

New 

Create a new layer at the bottom of the list. If any shells are picked, they will be moved into the new layer.

U 

Move the active layer up the list.

D 

Move the active layer down the list.

X 

Delete the active layer. Any shells in that layer will be moved into the top layer.

Move 

Move all picked shells into active layer.

Each layer has four widgets:

Active 

Click the gray button to make this the active layer. Shells not in the active layer are tinted gray and behave as if they are locked.

Name 

Change the name of the layer.

Pick 

Click the gray button to pick all shells in that layer. Click again to unpick the shells. This button also displays the number of shells in that layer.

Show 

Click the toggle box to show or hide all shells in that layer.