I belive it's referring to Decoration Disorder Disconnection, AKA DDD, which is another title written by Kinoko Nasu which was published in Faust magazine
Arika Ishizue lost his left arm in the A-syndrome related accident and hospitalized. After he left the hospital and went back to Shikura, he met a beautiful boy named Kaie Karyou who has no limbs and is living in a dark basement. The boy asked him to exorcise the "Demons" possessing A-Syndrome patients. Arika was involved in the bizarre cases and the fights with the "Demons".
Source: DDD on Typemoon Wikia
As the Wikia also states there are no connections between DDD and the rest of the Natsuverse, even though Mitsuru Kamekura from Kara no Kyoukai is refernced in it as the wiki states
it is simply a gimmick included by Nasu to cause readers of both to do a double-take while reading. It is actually a pseudonym taken from a character in a book in the DDD world.
However i do not know how being able to share the Holy Grail or not is related to DDD or how explaining it is "too adult" for Student 0. however given the case with Mitsuru Kamekura this could be the same thing with it being a gimmick by Kinoko Nasu and/or Gen Urobuchi to cause those who have read DDD to do a double take