I found this in a man page for
ls in solaris linux.
What does it mean when it says:
Dthe entry is a door?
A door is a special file in Solaris for bidirectional communication between processes (IPC). Doors provide client processes with a way to call remote code in a server process and fetch the result.
As @HBrujin stated, the Wikipedia article is a good entry point to doors (if that makes sense to you).
As explained by
ls man page, doors are identified by the letter