The Red sector was a sector of space, a region that became known by its political and diplomatic status as a red sector, a sector forbidden for overtures of communications due to political and diplomatic issues.
History and specifics
This Red sector of 24th century history was so referred due to the Bal Quonnot alliance with the Romulan Empire. A Romulan scientist's experiment led to a gravity disruption phenomenon known as the Constrictor, which devastated the planet PojjanPiraKot. The Pojjana civilization became distrustful of outworlders, but courted Federation alliance or Federation membership in order to alleviate their problems. Their expulsion of Federation diplomats in the year 2353 cause the sector to be declared a "red sector". (TNG - Double Helix novel: Red Sector)