This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
There are subcode flags in the Q channel for each track:
If set, the track contains data; if not, the track contains audio.
Digital Copy Permitted
Used by SCMS. Set to allow copies, clear to prevent them.
The Red Book standard allows four-channel audio, though very few
discs have ever been made that use it.
Set if the audio was recorded with pre-emphasis.
The last two are rarely used.