The ATX power supply uses 3.3, +5, -5, +12 and -12 volts, plus a common or ground. The +12 and -12 are used by one pin each, so I would guess that would be what you mean by the seldom used.
The 3.3 volt is what distinguishes the ATX from earlier power supplies, which only had +5, -5, +12 and -12, plus a common or ground.
+12v,-12v, +5v, ,-5v, +3.3v and the one seldom used is -5v
What are the 5 voltages produced by and BTX power supply? +12v -12v +5v -5v +3.3v
+12V, -12V, +5V, -5V, +3.3V -5V is seldom used
12V, 8V, 5V, 3.3V, and 2V.
+12V, -12V, +5V, -5V, +3.3V
+3.3v +5v -5v +12v -12v
-5v is seldom used because of the ATX12V 1.2 revision in 2003
+12, -12, +5, -5, 3.3 and ground (or 0).
the ATX standard supplies three positive rails: +3.3 V, +5 V, and +12 V
+12 volt , -12 volt, + 5 volt, - 5 volt, and + 3.3 volt.
12v, 3.3v 5v
ATX case is not used for all but it is still used for most.