So quickly browsing through the source code doesn’t really reveal much. From what I can gather, it’s either an unfinished attribute, or just completely useless.
It works exactly the same as the warp attribute, only it sends a packet to the client, which sets some values in the TempTileRec.
It uses the same arrays as the Key attribute, which opens and closes a “door”. What I’m guessing, is that this Door attribute was meant to show a layer on that tile, disappear when the door was opened.
If anybody could shed some light on it, that would be great, as it’s been sat in the source code since Eclipse Origins or even before, and it does absolutely nothing.