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The type of command you send to a Motor Controller differs between models. Some are designed to take a target position, others a target velocity, and still others a target torque (i.e. "effort").

So the difference between the effort_controllers and velocity_controllers is the type of command that the ros_control stack sends down to the hardware_interface::RobotHW layer. Typically the value is then passed directly to the motor controller, so the cmd type must match what the motor controller is expecting.