There are two critical criteria of a professional DJ mixer, That is having at least two separate outputs, one for the crowd ( a master output) and one for your headphones ( that only the DJ can hear. They both are used to reference the chosen audio signal . and they are separate so that a DJ can control what they hear in their headphones and have a different output that the crows can hear, both independent of each other. The reason is so that The DJ can cue the next song before they send it to the crowd.
The headphone output is controlled by the Headphone cue buttons that appear above the fader on each channel. This allows you to control exactly which channels go to the headphones. This is important because if channel 2 is playing on the Master output to the audience, you want to be able to hear channel 1 before anyone else, so that you can prepare to mix the new track in order to keep the music moving and constant.
There is also a headphone cue for the master fader and you will need to activate this if you want to mix in headphones. This is done by turning the mixing knob under the levels knob in the headphones section.
This will allow you to practice your mixing in headphones so that your neighbors wont hate you. It is also good for other incidences such as if you don't have a dedicated booth monitor, because if the speakers are too far away from you, there will be a time delay in your mixing.
This is similar to the effect of watching a soccer ball being kicked from far away. you hear the sound only after the ball has been kicked because the speed of sound is relatively slow. This is what it is like to experience a delay and it can be extremely harmful to your mixing. So if your speakers are far away you are best to reference the mix in your headphones (cans