This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
Its intended use was as a way to program the Holodeck and access the
ship's computer, as well as a virtual reality "safeword". In later
episodes, they just used the "exit" and programmed the computer by
voice. It is still around, recently seen in "Ship In A Bottle" [TNG].
The arch was how Moriarty first learned that he was a simulation, and
gained control of the ship in "Elementary, Dear Data" [TNG], and it
was seen in Star Trek: Generations as well.
 
Continue to: