Description of how the instrument is played, how the sound is produced. Anything about the way the instrument is played that is pertinent to composing - i.e. if low notes can't sound very quickly, or if it's difficult to jump octaves, or if the instrument is particularly nice in legato, due to its construction.
Also, discuss other techniques - pizzicato, harmonics, glissando, anything else that the instrument IS capable of.
AUDIO: Any of these secondary techniques being performed.