Pioneer Dj DDJ-FLX10 4-Channel DJ Controller for rekordbox and Serato DJ Pro (Black) - Manual (Page 52)

Part names and functions
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10 Headphones CUE button (master)
Press:
Plays the master sound through your headphones.
11 Headphones CUE buttons (channel)
Press:
Play the sound of the relevant channel through your headphones.
[SHIFT] + press:
Switches between EQ mode and Part ISO mode for the EQ knobs on the
channels that are controlling DJ software.

See: 9 EQ (HI, MID, LOW) knobs
12 Channel faders
Adjust the volume of the sound for each channel.
13 Crossfader assign switches
Assign the channel output to the crossfader.
[A] : Assigns output to [A] (left) of the crossfader.
[B] : Assigns output to [B] (right) of the crossfader.
[THRU] : Select this when you don’t want to use the crossfader. (The signals
don’t pass through the crossfader.)
14 Crossfader
Outputs sound assigned by the crossfader assign switches.
15 HEADPHONES LEVEL knob
Adjusts the volume of the headphones sound.
16 HEADPHONES MIX knob
Adjusts the monitor volume balance between sounds from the channels which
have the headphones
[CUE] button pressed, and the master channel.
17 SAMPLER VOL knob
Adjusts the volume of the sampler sound.