Double notches are most commonly used to indicate the direction a piece should be sewn into a garment (and normally indicates the back of a pattern).
For example, a double notch is used on a sleeve cap to indicate where the sleeve cap meets the back armhole (while the single notch on the sleeve cap indicates where the sleeve connects to the front armhole). In a side panel, a double notch is often used to show where the piece meets with the back panel of the garment. I also like to use a double notch to indicate the end of a zip (so the notch is differentiated from balance notches etc.) and can be quickly and easily identified.