The rear of the refit Enterprise‘s superstructure was given four large windows. It was only seen from the inside once, when Spock’s shuttle approaches the ship in The Motion Picture. But this is where the officers’ lounge scene between Kirk, Spock and McCoy later in the movie was supposed to take place.
Shane Johnson writes in Mr Scott’s Guide to the Enterprise (1987) that the reason the scene was filmed in a smaller room is because the film had already run over budget.
“The briefing room scene was rewritten for an ‘officers’ lounge’,” Johnson told Trekplace‘s Greg Tyler, “and a set was created somewhat hastily using existing ‘rec deck’ components.”
Andrew Probert, who had been involved in redesigning the Enterprise for The Motion Picture, rationalized the scene by suggesting that the small lounge could actually be a secluded area within a larger officers’ lounge. That’s also the approach Johnson took in Mr Scott’s Guide to the Enterprise.
In canon, the layout of the rest of Decks 2 and 3 is unknown. Mr Scott’s Guide suggests Deck 2 is home to security facilities wile Deck 3 has a large officers’ mess and lounge surrounding a kitchen, which is seen in Star Trek VI: The Undiscovered Country.
The forward end of the superstructure features a series of details on the outer hull which have not been officially identified. Franz Joseph’s deck plans for the original Enterprise placed science labs in the front section of Deck 2, suggesting these may be sensor pallets.