CD EXTRA is the latest type of 'Enhanced CD'. Enhanced CD is the umbrella-name given to all types of CD audio (red book) which have been combined with CD-ROM (yellow book) data. There are three main types of Enhanced CD:
Mixed Mode and Pre-gap CD Layout Mixed mode or track one CDs put the yellow book data in before the first audio track of a red book CD. The first audio track then becomes track two. If you play this type of disc in your CD player you must 'skip track one'. Mostly the data will not be interpreted by the audio player and track one will be silent. However, some audio machines will play the data - causing a noise which is dangerous to both speakers and ears.
Pre-gap or hidden data CDs put the yellow book data in index zero of track one on a red book CD. The audio index then starts in track one of track two. Most audio CD players will ignore the data, many (but not all) CD-ROM drives will find the data. It is theoretically possible for a person to 'rewind' the CD into the data and create the same sound problem as with track one, although this is unlikely to occur by accident.
CD-EXTRA Layout CD-EXTRA has an official definition (blue book) and stores the red book audio in the first session on the disc and the yellow book data in a second, completely separate session. Since an audio player is a single session machine, it is only able to access the first (audio) session. Most new CD-ROM drives are multisession which means they can see the data session as well as the audio session. CD-EXTRA was originally known as CD-PLUS. Although CD-EXTRA solves most of the problems encountered with the earlier Enhanced CDs, it is a new format and not immediately supported by all CD-ROM drives.
For the MMB users: Using Mixed mode the first audio track is track 2. Using CD_Exttra the first audio track is track 1 !