Starship Mission Assignments chart on the bridge of the Enterprise
Sector 21399 was a region of space.
In 2293, the Federation starships USS Whorfin and USS Korolev were assigned to missions in this sector. The Whorfin was on deep space exploration, while the Korolev was on a diplomatic mission. (Star Trek VI: The Undiscovered Country set artwork)
This sector was only mentioned in writing.