Channel description of the Non-Diegetic Audio Box

The Non-Diegetic Audio Box description is a little confusing and leaves too much to the implementation.

> When present, provides additional information about the non-diegetic audio content contained in this audio track.

What does this mean? What is "additional information", given that the box only contains a version field? I would understand "if present, means <...>". but that's not what it says.

> audio which should remain unchanged by listener head rotation; e.g., narration or stereo music.

Does this mean the layout can be mono or stereo? Or only either one is supported? If stereo is supported which channel is the left one and which is the right one? Does it have to be read from `chan`? If so, it should be mentioned in the description.