I think on macOS and iOS it only works in actual image files, but this tool predates that by a year or two. This does the same thing but doesn’t require an image file; you just press the shortcut on your keyboard, draw a box over whatever’s on your screen that you want, and the text in the box goes on your clipboard. I think it’s effectively taking a screenshot but not saving it to disk, so you don’t have to clean those up later.