It indicates what scenes they are for, but not necessarily that they were used in the final cut. However, having that tag definitely makes the piece more desirable. You can't honestly say something is screen used unless you can screen match it, which means you can identify marks or imperfections that definitively indicate that the piece you have is the one on screen. Or, if you received the costume from someone in the production with direct knowledge of wardrobe used in that scene and they claim it was; that's also good evidence for screen use.