This is a dead thread but because the question was never properly answered:
Each power bus/ribbon cable on the uZeus (or any Eurorack power supply) contains wires carrying all of 0V, 5V, 12V, and -12V. This is what TipTop meant when you asked which bus to use: Either one, because both buses contain all of these voltages. There isn't a "positive" and "negative" bus in that way. Sometimes people may refer to any one of the voltage supplies within the ribbon cable as a bus, which is reasonable if you're drawing up a schematic for a single module, but less so if you're connecting multiple modules in a case. Personally, I'd use "Power Bus" to refer to the entire ribbon cable's worth of power supplies.
It is important to always connect your modules to the ribbon cable correctly: There should be some indication which side of the module's power connector lines up with the red wire on the ribbon cable (which indicates the -12V side of the ribbon). If not, call the manufacturer and chastise them for not making that clear.
A common use case for the -12V supply is amplifiers: if you want to amplify a signal that goes below 0V, it's helpful to have a negative-voltage power supply to make that happen. This could mean VCA modules, or just op-amp components (which appear in almost any analog circuitry).