The rear of the refit Enterprise 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.
Lora (then 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 Motion Picture had already run over budget.
“The briefing room scene was rewritten for an ‘officers’ lounge’,” Johnson told Greg Tyler of Trekplace, “and a set was created somewhat hastily using existing ‘rec deck’ components.”
Andrew Probert, who was part of the team that redesigned the Enterprise for the film, rationalized the scene by suggesting that the small lounge could be a secluded area within a larger officers’ lounge. That is 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 and 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.
Sources for this story include: Shane Johnson, Mr Scott’s Guide to the Enterprise (1987) and Greg Tyler, “Interview with Shane Johnson,” Trekplace, 2001