PC's use specific voltage and its important that these voltages do no deviate to far from spec. A 5% deviation is allowed for. Your Rail, which are for power, are: molex, pcie, and sata. They all use 12v and 5v. A true sata power connector will also provide 3.3v but I dont know of any device that actually uses it.
Each rail is also supposed to be independant of each other but sometimes testing reveals this is not the case. And not that a single rail PSU is a bad thing in itself, it all depends on what you need or planned for.