Flight Detent: The first sentence of your post explains it.Anytime your altitude is based upon standard setting of 29.92 inches/1013.2 hpa then it is expressed as a "flight level" irrespective of what follows is metric or imperial.You can find the explanation covering this in the Jeppesen Manuals.As is the case a lot of the time and all over the world, in practice many pilots/controllers dont always use correct phraseology over the radio, very common in China airspace where they largely omit the "flight level " part....Pete.