What is a "running title"? I'm about to submit a paper and the format requires a running title in addition to the actual title. What is it for, and is it supposed to be longer or shorter than the actual title? Should I abbreviate the title, or take the opportunity to convey another message that could not be included in the main title?
Running title: "the title or abbreviated title of a volume printed at the top of left-hand text pages or sometimes of all text pages"

It allows readers to determine which paper they're looking at just by glancing at the top of the page.

If your actual title is already very short, use your actual title. If your actual title is not very short, use an abbreviated version of it as the running title. (The maximum length is often set by the publisher; for example, APA style sets the maximum length of the running head to 50 characters, APS style allows 55 characters, etc.)