Lets you select an animation layer where you want to add the scale constraint. See Create scale constraints for more information.
When on, the layer you select in the Animation Layer drop-down list is automatically set to Override mode as you add the constraint to the animation layer. This is the default and recommended mode. When off, the layer mode is set to Additive as you add the constraint. See Add constraints to animation layers for more information.
See also Animation layer modes in the Animation guide.