This function turns on/off muting of the selected part (Drums / Vocal / INST) of the loaded track.

When Part Solo is set, only the selected part is output.

For details, refer to the Instruction Manual > [Part names and functions] > [Deck section] > [30 ACTIVE PART DRUMS button], [31 ACTIVE PART VOCAL button], and [32 ACTIVE PART INST button].

You can read the Instruction Manual [here].