Creative Image Editing in Photoshop - Black and White in Photoshop CS4 and Photoshop Lightroom - page 201

3. Reviewing changes
Because the adjustment layer sits above the original image, reviewing
image changes in a before and after state can be easily seen. Clicking the
eyeball icons next to the adjustment layer allows you to turn the visibility
of each adjustment on and of and make decisions accordingly.
4. Layer options
Adjustment layers, as image layers, also provide layer options including
opacity, blending options, masking, selective editing and the ability to
transfer the adjustment layer to other documents.