The Inon 67mm Mount Single Mount Lens Holder DP (think "deep") provides a place to store either the Inon UWL-S100 ZM80 Wide Conversion lens or the Inon UFL-M150 ZM80 Underwater Micro Fisheye lens. These two lenses are "deeper" than other wet mount lenses and might not fit traditional 67mm lens holders. (The rear of the lens protrudes beyond the threads of the lens adaptor.)
It allows you to quickly switch between wide-angle and close-up lenses for more shooting flexibility without fumbling around trying to get your lens into or out of a BC pocket.
The caddy comes with one lens mounting ring, and the hardware to attach if to a strobe arm (ULCS or INON or Nauticam), or the INON Grip Handle.