The supply voltage for a circuit is often given as V plus a double-letter suffix. The double letter is usually related to the lead of the transistors that are commonly connected to that supply or to a resistor that connects to that supply.
Examples: VCC is a positive-voltage supply and the collector terminal of bipolar transistors is connected to the VCC supply or to a load which connects to VCC. VSS connects to the source terminal of a FET, etc.
V+ and V- are also common ways to refer to a supply voltage.
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