CD-Extra - Multi Session Audio CD

CD Extra or Enhanced CD is a standard that covers the following CD layout :

First Session : All Audio Tracks
Second Session : One or more data tracks which contain the File System(s)

The reason for this is that Audio Players will only see the first session (Redbook standard) and so will not bother with the data in the second session.  An OS however will look for the File System and will find this in the data track of the second session.