The 1.25" Drop-In Eyepiece Projection Camera Adapter is used with SLR cameras, and does two things.
First, it can be used without an eyepiece in place, providing more back focus for scopes that would otherwise have problems with prime focus imaging with the LC1020 1.25" Prime Focus Camera Adapter. When used for prime focus imaging, you'll be able to place a 1.25" astronomy filter upon the front.
Second, it can be used together with an eyepiece inside of the adapter to achieve very high magnification. This is great for lunar photography. Any 1.25" eyepiece with a maximum diameter no larger than 36.8mm can be used.
In either case, you'll still place your camera's specific T-mount upon the rear of this adapter, and affix your SLR camera upon the rear of the T-mount.