How to identify power supply rails ?
Many ATX power supplies have multiple "rails" to limit the amount of current through each wire so that cables would not get dangerously hot. This is most relevant to the +12V yellow wires coming out of the PSU.
To identify rails, look closely at the cables coming out of the power supply. While some manufacturers put actual labels on them for the multiple rails, most also put a stripe of different color on wires to identify different rails. The most common identification is solid yellow for rail1, yellow with a black stripe for rail2, and yellow with different color stripe for additional rails, where applicable.
Some power supplies do not have multiple rails.
On power supplies with two rails, it is possible that only the ATX12V/EPS12V is connected to +12V2 rail, and all the rest of the cables are conected to the +12V1 rail.