Strangely, the door cut directly through the horizontal intermix shaft without any obvious cutout or opening, perhaps suggesting the door also interrupted the flow of warp plasma. Additionally, it is unclear what the door was designed to isolate.
Main engineering aboard the Galaxy-classUSS Enterprise-D was equipped with at least two isolation doors. The outer isolation door served to separate the warp core and engineer's office from the rest of engineering, the inner door only separated the warp core from the rest of engineering, in an emergency, the engineer's office would still be accessible.
Isolation doors were also used in the nacelle tube control rooms aboard Galaxy-class starships. The isolation doors could be opened with the protection of a one-way force field. The doors, however, could only remain open for ninety seconds in order prevent force field degradation and the release of drive plasma. (TNG: "Eye of the Beholder")
Some starships, like the Intrepid-class lacked isolation doors, as their engineering sections were extremely open and were separate from the rest of the ship with no open corridors leading to the area. (VOY: "Day of Honor", "Waking Moments")
The isolation door also served another purpose in TNG: When an extended corridor scene was needed, the "pool table" in engineering was removed and the door was brought down to hide the warp core. The two "plugs" that hid the large corridor sets were removed and the master systems display was hidden behind a wall giving the appearance of a lounge within the corridor.