I'm assuming you're talking about the plug on the side of the turntable. It allows you to connect a drummania pedal and use it as an alternative scratch.
Please read the stickied FAQ, it was intended for the pedal for a home version of Beatmania III that was never released.
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Originally Posted by tomokosasaki
GOLD will come with 2nd conclusion!! Just have some trust. I can't always tell my source! But, in the end, you'll see i was right and this conversation won't matter anymore.
Beatmania controller question 
