The shell room was the room in a TARDIS where the plasmic shells were stored. The chameleon circuit then picked the most suitable shell from the room, and applied it as the ship's exterior. The shell room, ten metres wide, was more like a seemingly unending corridor displaying a large variety of plasmic shells for different times and places.
In the case of the Doctor's TARDIS, whose chameleon circuit was broken from the start, the room was useless and the Doctor never used it. The Fourth Doctor and Romana II were bewildered to find a message on the TARDIS scanner saying, "Warning: Shell room full". This lead to the two discovering the TARDIS' final exterior before becoming a police box: a post box with a misplaced letter inside. (PROSE: The Little Things)
Lord Roche's TARDIS shell room did not contain any Earth-style aeroplane plasmic shells, so he was forced to use a model of another time and planet. High up in the sky, though, no one noticed. (PROSE: The Suns of Caresh)