The eyepiece holder which comes standard with some Schmidt-Cassegrain telescopes threads on to the rear cell of the optical tube and reduces the cavity to accept 1.25" accessories such as star diagonals or tele-extenders.
In the tele-extender instance it will hold the eyepiece for eyepiece projection. Schmidt-Cassegrains which come supplied with electric focusers don't have this as a standard accessory.