The traditional mental image of a fastener is a bolt with a body, threads, and a head of some kind. Not all fasteners include a head, however. While the main purpose of threaded fasteners in automation is to join two or more rigid bodies, fasteners are also used for other purposes, such as locating, retaining, and adjusting.
When a fastener no longer requires the bearing surface of a head to hold a part in tension, a headless fastener may become the better choice.