A placard, I'm sure you have all seen them, they come in various shapes and colors. There is typically a hard plastic, for lack of a better term, with a number on it. And the only purpose that it truly serves is for primarily me to recognize a photograph, what number of evidence that I'm picking up.
Any given scene shell casing or a blood drop somewhere. If you find a picture of this shell casing 100 pictures later of a separate shell casing, it would be nearly impossible for you to determine which was number one and which was number 200.
So a numbered placard is just a reference for my report writing and my evidence collection of what I've just took a picture of and I can reference that in my evidence.