A Deck is a location aboard the JMC mining spaceship Red Dwarf. A Deck was located at the top of the ship, containing the Penthouse Suite. (Camille)
It is also presumably contained the original Observation Dome (Better Than Life, Thanks for the Memory), and other structures seen on the top of the ship in various ship fly-bys.
It did not contain the captain's quarters; that was located on Floor 341. (Only the Good...)