The situation room was a small section located at the rear of the bridge aboard some NX-class starships in the 22nd century. The room was a precursor to the briefing room aboard Federation vessels, where senior officers could hold discussions on subjects such as the progress of a mission. (ENT: "Broken Bow", et al.) When it was not used for briefings, the situation room was often manned by several crew members. (ENT: "Zero Hour")
The situation room was marked by several computer displays and stations (including the environmental controls), with the monitors projecting status displays, and a small table with swing-out chairs built into its structure. A table graphic was displayed on the table and a plasma screen was used as a back wall monitor in the room. (ENT: "Broken Bow", "Rogue Planet", "Chosen Realm", et al.) The monitors in the situation room could be turned off remotely, from the science station. (ENT: "Rogue Planet") The room was additionally accessible through a doorway located on the port bulkhead, through which there was an entryway equipped with an intercom. (ENT: "Acquisition", "In a Mirror, Darkly")
In the mirror universe, the NX-class ISS Avenger did not have a situation room, but a wall and a bank of computer monitors in its place. (ENT: "In a Mirror, Darkly, Part II") NX-class starships that did have a situation room include Enterprise NX-01, Columbia NX-02, and, in the mirror universe, the ISS Enterprise NX-01. (Star Trek: Enterprise)
While Captain Archer visited Columbia in 2154, a worker wearing a faceplate was welding the central support column of the situation room's table onto the deck, and the bulkheads were hidden behind plastic drapes. (ENT: "Home")
In 2155 of the mirror universe, the situation room of the aforementioned ISS Enterprise was almost identical to the equivalent rooms of the prime reality, although the port-side door was emblazoned with the emblem of the Terran Empire. (ENT: "In a Mirror, Darkly")
History of a situation room
In the script for "Broken Bow", the situation room was said to be included as part of the NX-class bridge, within sight of the bridge stations. It was also described as "a tactical center off the Main Bridge, filled with various read-outs and a large table graphic." 
The design for the situation room obviously took cues from the aft section of the USS Defiant from Star Trek: Deep Space Nine, also designed by Herman Zimmerman. He, however, likened it to such comfortable retiring rooms located off the bridge as the observation lounge on the Galaxy-class USS Enterprise-D from Star Trek: The Next Generation. "By putting it in the bridge itself it gives you the opportunity to have quiet officer meetings close to the center of command," Zimmerman stated. "And it also gives extreme depth to the look of the set: you can shoot both from the viewscreen through the bridge to the Situation Room, and from the back of the Situation Room all the way to the viewscreen." (Star Trek: Communicator issue 135, p. 61)
The situation room's plotting table was designed by illustrator John Eaves, who described it as "a really cool piece" and "really a great project to work on." Recounting the table's genesis, Eaves exclaimed, "Herman just said: make us a nifty table to have meetings at!!!" Though this item of furniture is slightly similar to another table – used in engineering of TNG's Enterprise-D, an item often nicknamed the "pool table" – Eaves took no inspiration from that table in coming up with the design of the NX-class plotting table.
John Eaves initially sketched two variations of the table, a pair of drawings that described the item of furniture as a "conference table". Whereas the first of these illustrations had a raised plotting surface, this area was flat and recessed in the second picture. Once the first suggestion was approved, Eaves produced a final drawing that took cues from this image while also adding more detail. The table underwent more alterations between this couple of similar illustrations and the final product. Explained Eaves, "The early ideas were of the same layout [as the final table] except the rounded table top had plotting rules and sliding scales that would be able to move and plot across the entire surface. The curved surface would be made of milk plex and the images would be projected from underneath so it would act like a big hands-on cartographer’s mapping system. This was axed to be instead a clear covering with a flat screen system under glass, and the sliding scales were thought of as too out of date." Eaves also clarified that the evolution away from the idea of the table incorporating a projection setup was due to the amount of flat screens used elsewhere aboard the NX-class.
The plotting table was built by Construction Coordinator Thomas J. Arp's set construction team and was detailed by Michael Okuda and Anthony Fredrickson. The size of the table was dictated by how big its screen was, so it turned out smaller than originally conceived. Ultimately being of the opinion that the table "has a definite sub feel," John Eaves found the building of the table to have been "beautifully" achieved and thought the process of Okuda and Frederickson adding their finishing touches was "stylishly" done. Eaves was disappointed, though, that he couldn't take the actual table home with him as a keepsake, after the making of Star Trek: Enterprise ended.
The situation room's rear screen was designed by Michael Okuda and James Van Over. Eaves described their collaboration on this screen as "awesome work".  Footage of Van Over designing a computer readout to be displayed on the screen can be seen in the documentary On the Set, available in the ENT Season 1 DVD and Blu-ray.