Generally, a head unit can have preamp outputs in one, two, or three sets.
If the deck has one set of preamp outputs, these are usually called rear outputs. Meaning that if you fade the speakers to the front, the rear speakers and the rear preamps will fade out.
If the deck has two sets of preamp outputs, there is usually a front set and a rear set. The front speakers/preamps will fade when you turn the fader to the rear speakers/rear preamps and the rear speakers/preamps will fade when you turn the fader to the front speakers/preamps.
If the deck has two sets of preamp outputs, there is a front set and rear set that act like I described above and a third set. The third set is usually a subwoofer or non-fading preamp output. The subwoofer/non-fading preamp output will not fade if you use the fader to fade from front to rear. Depending on the deck, there may also be some level of sub control, meaning you can turn the sub up or down or control the phasing of the sub.