The 12-2 or 12-3 is used if you want to combine the receptacles and lights and fan for one bathroom on one circuit (U.S. locations requiring 20 amp bathroom circuit.)
When you have a double wide 20 amp breaker you can use one 12-3 instead of two 12-2s. Just be sure that the breaker is positioned in the panel so you get 240 volts across the two breaker screws. (It's still 120 volts from hot to neutral.) One single wide (tandem) double breaker may not be used with the 3 conductor cable.
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