The Vixen DG-DX Series Adapter Rings are used in combination with either the #3918 DG-LV DX Digiscoping Adapter or the #37221 DG-NLV DX Digiscoping Adapter, completing the puzzle so that a digital camera which accepts filters can be affixed to the rear of a telescope with an attached eyepiece.
The 28mm Adapter Ring has, as the name would imply, a 28mm male threading on one end. This means that it is intended to use with digital cameras accepting 28mm filters. In certain cases combining longer eye relief eyepieces, it may be possible to add a step-up ring to the mix to use a camera with <28mm filter threads. But because the step-up ring adds some 'stack depth' to the mix, doing so can extend the camera too far from the eyepiece to make successful, vignetting-free images. The only way to know is to try.