A Deck is a location aboard the JMC mining spaceship Red Dwarf.
A Deck was located at the top of Red Dwarf, 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...)
A Deck was seen a number of times in Series XI-XII (as can be seen on the corridor signage), although the parts seen are not dissimilar to other parts of the ship, say B Deck.
The new sleeping quarters used by the boys from the Dwarf from Series X onwards was located close by to the top of the ship, either on or near A Deck or B Deck. The Science Room used by the crew in Series XI-XII was located on G Deck, 229 floors below the sleeping quarters. (Skipper)