Add a shaped section to one of the corners of an element/area, and use it to show some other element.
If you want to call something out to a user, you can add an area to one of the corners of an element or area of the page. I see this most often in the top right corner. It might remind people of the area on physical packaging where a guarantee or "As seen on TV" message is shown.
The area does not have to be triangular like the one in the example below. It could be square, for example.