The abbreviation d.r.f or drf stands for de.rec.fotografie, a newsgroup on Usnet German. Photography enthusiasts there created a wiki-style FAQ site called drf-FAQ. In there you can find various interesting pages. The one that got my attention was their Nikon i-TTL page (Google translation, see original German language page).
The page starts with brief history and the basic principles of the Nikon iTTL system, then it goes through iTTL capabilities of various cameras and flashes (including both Nikon and 3rd party products). It also has a section about the limitations, problems, and workarounds of Nikon iTTL system.
DPTnT is mentioned on the page for our contribution in understanding the working principles of Nikon creative lighting system (CLS) advanced wireless lighting (AWL).