Crossfader, Line Fader & Gains?
To someone who is not a DJ, the difference between cross faders and line faders is pretty clear cut. One of them controls the mixing from left channel to right channel, while the up faders (line faders) control the volumes of each channel individually. In real world usage, the purposes of each are not as defined, and in fact, many DJs would prefer to NOT use them as common sense might predict. A scratch DJ takes the cross fader and uses it more like a switch that turns the sound on and off – rarely does he or any other DJ use the cross fader to sweep from one channel to another.
The up faders (line faders), also have some of same neat tricks that can be done to them. Sometimes they can be used to fade extremely sharp, without actually cutting the sound on and off, so it gives it more or less of an attack. A clever DJ will combine the gain, channel faders, line faders, and even the cross faders to give off the appearance of doing many different things, when it could easily be achieved by using only one of those. It speaks for showmanship rather than just purpose.