All items have a Lock option where you can set that an item is locked. When an item is locked it will no longer have the resize circles on each corner and will be outlined by a gray rectangle.
Locking Items is a useful thing to do as the system uses automatic snapping to align controls. Having too many controls can mean that you align to a control you didn’t mean to. Locking can help with this. When doing swim lane diagrams a good suggestion is add the swim lanes to the diagram and then lock them.
The Lock and Unlock items locks multiple items that are selected in one action. You would use it in conjunction with the Multi-select button or the Selection Rectangle.