Somewhere in the middle of the development of the ATX power supply specification somebody thought there was a maximum power per rail restriction before you created the potential for a fire. And, as a result, consumer power supplies started having more than one "rail". Each rail has a maximum total power below that magical limit. A 1000 watt supply would have 4 "rails".
Now this in itself isn't any big deal as long as you know which plug is on which rail. In the beginning it was easy. A dual rail supply had one rail dedicated to the CPU and one rail serving everything else. Easy. The 4 pin plug going to the CPU had half the capacity.
But then we started getting high power GPU's, and people using high power GPU's with low power CPU's. Mining is a great example. Miners don't want to reserve / allocate / waste 250 watts of their 1000 watt supply for the CPU, especially since they are going to put a low power processor in there. But, with a traditional multi-rail supply you are stuck. That is unless you do some fancy connector jiggermary.
A 4 rail supply like linked by the OP is likely (and I am guessing here) to have 1 rail dedicated to the CPU, 1 for everything but PCIe and two split across the PCIe power plugs. Yes, that's right. You bought a 1000 watt supply for your GPU's but half of it is unusable by the GPU's. Ok, I exaggerate. You can recover some of that power from one of the rails through the PCI backplane and a bit more from the molex connectors.
Personally, the safety of a multi-rail supply has been debunked. It is no longer a requirement of the ATX power supply specification. I for one see no reason to buy a multi-rail supply. It just makes you think more about what you want to do and occasionally stand on your hands and touch your toe to your ear to get the supply to run all your gear. I don't need those contortions. Thus I recommend single rail supplies. You can be smart or you can be lazy. I choose to be lazy
If you are the average joe consumer who is building himself a hot rod computer with a decent processor and a decent video card and a bunch of peripherals then a multi rail supply will work just fine. Your i7 will be perfectly happy on one of the rails, your video card will be perfectly happy on another and one to run the rest. No problems at all.
But, if you are trying to run three video cards, all of which want 300 watts each, then using a multi rail supply that dedicates 250 watts to the CPU leaves you 750 to run 900 watts worth of GPU. You can get a maximum of 150 of that 900 from the pci backplane leaving you needing 750 watts from the PCIe power plugs which are spread across 500 watts worth of capacity. Not a good situation to be in.
Now, realize that this is purely speculative about what the supply manufacturer did and purely theoretical. You may, and many do, get away with ignoring the specified individual rail limits. But I only buy single rail supplies.
Does that help?
