The Golden Rule
The Spacesim motto, Nothing goes in or out, represents all that Spacesim is about: mission integrity, realism of simulation, and suspension of disbelief. Simply put, the astronauts must believe they are on another planet or moon in the solar system, and receive no external inputs from the real world. Most importantly, nothing may enter or exit the simulation room throughout the mission for any reason barring a real-life emergency. To a lesser extent, this represents the need that the astronauts forget their real lives for a few days, unplug from the Internet and society, and receive all their information exclusively through Mission Control.
Historical Breaks from The Golden Rule
During several missions, The Golden Rule was broken, and allowed Spacesim exactly why it was in place. During several missions, class notes were passed through the Powerbox in order to allow astronauts to remain up to date on their class work. Unfortunately, this break from Mission Integrity diminished the effects of the simulation, in effect providing a link between Earth and the Habitat, despite the Habitat being several thousand kilometers away.
An even worse break from the Golden Rule was the transference of an X-Box through the Power Box into the Habitat. This essentially ruined the mission by the same method described above, and by sapping attention away from the mission and towards video-games.