Unlockables:-

Music Select BGM customization - In the Customize section of the game, you can also replace the music select system music with a composer customized music or older Style system music. To unlock the previous system music, you have to play a song from an older Style. To unlock other DJ style music, you have to play a song that they composed (it can be an old song as well). You do not have to pass or finish the entire song to unlock.

in the Sky / Ryu Pass 5 Dan courses
Just a Little Smile / Sota Fujimori Pass 5 Dan courses
Sayonara Heaven / Nekomata Master Pass 5 Dan courses

IIDX RED BGM is unlocked by playing the remix of the IIDX RED system ending theme, CaptivAte ~jouka~.

10th Style Play any song from 10th Style (i.e.One More Lovely, Love Is Eternity)
11th Style -IIDX RED- Play CaptivAte ~Jouka~
3rd&4th Style Play any song from 3rd or 4th Style (i.e.Starmine, 5.1.1., Dr. Love)
5th Style Play any song from 5th Style (i.e.Over The Clouds, V, Sync)
6th Style Play any song from 6th Style (i.e.Blueberry Stream, VJ Army, Kotti Wo Muiteyo)
7th Style Play any song from 7th Style (i.e.Tomorrow Perfume)
8th Style Play any song from 8th Style (i.e.Please Don't Go)
9th Style Play any song from 9th Style (i.e.Abyss -The Heaven's Mix, moon_child)
dj Taka Play any song from dj Taka (i.e.Tomorrow Perfume, Inazuma, Rage Against Usual)
L.E.D. Play any song from L.E.D. (i.e.Love Is Dreaminess, Dawn -The Next Endeavour-)
Tomosuke Play any song from Tomosuke (i.e.Tizona d'El Cid)
Yoshitaka Play any song from dj Yoshitaka (i.e.Catch Me)

Unlocking Hidden Songs - Pass five Dan courses in Class mode to unlock Happy Sky hidden songs.

Extra Stage interface in Free Mode - To enable the Scream Squad song select interface during FREE mode (as is usually only available in ARCADE mode), before you enter the song select screen, hold the START and SELECT buttons down. You can hold the buttons down when you hit "START GAME" from the Gameplay Menu or after you select "SONG SELECT" from the Judgement Screen. You can also hold [START] and [SELECT] while the song select screen is loading (but wait too long, you'll get the normal interface).

Play Scripted Connection (long mix) - To play Scripted Connection (long mix), choose Class mode. Scroll to the bottom, and continue to scroll down until "Con'tion" is heard. Select the course, and instead of playing the Kyu course, you will play Scripted Connection (long mix).