In which situation is a vehicle in “neutral” gear?

A vehicle is in "neutral" gear when the engine is disengaged from the drivetrain. This means that even though the engine may be running, it is not sending power to the wheels. In this state, the vehicle can roll freely without the engine influencing its movement. Engaging the clutch in a manual transmission vehicle allows the driver to shift into neutral, effectively disconnecting the engine from the wheels.

In contrast, if the engine is running at high RPM, it indicates that the drivetrain is likely engaged, creating a direct connection between the engine and the wheels. When the engine is connected to the drivetrain, the vehicle will respond to throttle input and move accordingly. While engaging the parking brake may prevent the vehicle from rolling, it does not alter the state of the transmission with respect to whether it is in neutral or not.