Philips CDD522 Operating Instructions Manual page 39

The only difference, at disk-level, between a CDROM-XA
and a CD-|! track is that the latter is not listed in the TOC
of the disk.
F. CD-| Ready Type | disks are special CD-DA disks. CD-
DA disks have a 2 second of audio silence in the pre-gap
before the first track. Audio players skip this pre-gap.
The CD-! Ready Type | disk has an extended pre-gap
where CD-I information is hidden. An Audio player will
normally not notice this CD-| information, but with a CD-I
player extra features can be accessed this way.
Possible features are the display of lyrics or credits during
song-reproduction, the display of album-covers or
interviews of the performer in compressed audio
(remember, sector Type 5).
Unfortunately, older CD players did not mute the pre-gap.
If you ever heard CD-| data through loudspeakers, you will
understand this CD-1 Ready Type | was not useable.
So...
The CD-DA tracks are placed in a first session and the
CD-I tracks are placed in the next session.