No insignia was created for the Defiant crew's tunics; however, in creating "In a Mirror, Darkly", the wardrobe department came up with something new: "a variation on the standard Starfleet arrowhead, tilting it and adding an extra angle to its shape." In the original TOS episode, some Defiant crew members may be seen wearing the Enterprise assignment badge. See more
It's obviously a stand-in for Scott, and not James Doohan
, whom the mad crewman jumps down upon in the Engine Room. See more
I don't understand what's so special about this region of space.
Picture it this way, Mr. Chekov: we exist in a universe which co-exists with a multitude of others in the same physical space. At certain brief periods of time, an area of their space overlaps an area of ours. That is the time of interphase, during which we can connect with the Defiant's universe.
Mr. Spock? We WILL be able to retrieve the Captain at that time... won't we?
Yes. However, the dimensional structure of each universe ...