Why are not all wires used in the T4 Pro thermostat installation?

Not all wires are used because different HVAC systems require different wiring configurations. The T4 Pro supports various system types, including single transformer, dual transformer, heat-only, and heat/cool systems. Some terminals may not be needed depending on your specific system setup. The optional THP9045A1023 Wire Saver Module can also reduce the number of required wires by combining functions.