The orange '120/240' voltage switch is for power supplies that do not have active PFC (power factor correction). The switch toggles a voltage doubler circuit. Since they don't have PFC, the power factor can be 0.6-0.8, which means that if the active power is 100w, the reactive power can be as high as 167 VA. This requires thicker cables and stresses the utility company's primary-to-secondary transformer.
This is an
outdated design that is over 10 years old. Some OEM's like HP/Dell don't care because these older PSUs can be cheaper for them to buy in bulk, and a Dell Optiplex uses < 200w anyway. Never buy a PSU with a voltage switch, especially for mining. The minimum requirement should be active PFC, 80+ Bronze and 18AWG or thicker wires for the PCIe cables. I always buy 80+ Gold, though.
Power supply with the switch:
Raidmax XT. Do not buy!
Here's a list of power supplies I would consider:
80+ Gold or higherI also like server power supplies like the HP DPS-750AB/RB or a few Dell ones that breakout boards can be found for.