引用自
http://www.playtool.com/pages/psuoverload/overload.html

The ATX spec defines the use of each 12 volt rail for dual 12 volt rail ATX12V PSUs. The two rails are called 12V1 and 12V2. 12V2 is used to power the CPU through the 4 pin 12 volt power cable. 12V1 is connected to every other 12 volt cable. At least that's the standard. Not everyone obeys it. As far as I know, all dual 12 volt rail PSUs connect 12V2 to the 4 pin 12 volt power cable. And most PSUs connect the 12V1 rail to all their other power connectors. But some PSUs apparently use 12V2 to power some cables which according to the spec are supposed to be powered by 12V1. Unfortunately, they usually don't tell you which power connector is hooked to which rail.

簡譯一下:
ATX定義了雙線12V的ATX12V PSU使用
稱為12V1 and 12V2
12V2一般透過4 pin線路提供CPU用電(專用)
12V1則負責供給其他所有12V線路.....至少這是規格標準
但並不是所有廠商都遵循此標準....但大部分作者所見的PSU都是如此
但似乎還是有些PSU用12V2去分給其他該用12V1的線路,
可惜,通常他們都不會告訴你哪一條線路是用哪路的12V(也就是狼大所說,要拆才知道)