A split reel is used when handling film stored on a core. In exhibition settings, they are used to transfer film from storage on cores to show reels for projection. They exist for all film gauges.
Split reels are not suitable for projection. The two halves of a split reel are only held together by the threaded spindle in the center of the reel, and they can easily loosen and separate if used on a projector, leading to catastrophic film damage.