In the beginning of the Undiscovered Country, Hikaru Sulu has been promoted to captain and commands the starship Excelsior. Where was Styles, the previous captain during this time?
Canonically, it is never made clear. He may have died in the line of duty, transferred to a different ship, been promoted, or some other option. However, the non-canon novel Star Trek Excelsior: Forged in Fire addresses the question directly. In that book Styles is killed during a bombing of some Klingon-Federation peace talks on the planet Korvat in 2289, several years before the events of Star Trek VI: The Undiscovered Country.