Command format passed to hardware_interface by DiffDriveController [closed]

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Under my controller_manager I have a DiffDriveController and a hardware_interface with two Joints (wheels). How do I know what type of command is sent by the DiffDriveController to the cmd arguments of the JointHandles in my hardware_interface? Is it linear velocity, degrees per second, radians per second, or RPM? Can the command type be adjusted somewhere? Is this cmd argument the same as the publish_cmd published by the DiffDriveController?

Snippet from the constructor of my hardware_interface:

    hardware_interface::JointStateHandle left_joint_state_handle(LEFT_JOINT_NAME,
                                                      &pos[LEFT_JOINT_IDX],
                                                      &vel[LEFT_JOINT_IDX],
                                                      &eff[LEFT_JOINT_IDX]);
    jnt_state_interface_.registerHandle(left_joint_state_handle);
    hardware_interface::JointStateHandle right_joint_state_handle(RIGHT_JOINT_NAME,
                                                      &pos[RIGHT_JOINT_IDX],
                                                      &vel[RIGHT_JOINT_IDX],
                                                      &eff[RIGHT_JOINT_IDX]);
    jnt_state_interface_.registerHandle(right_joint_state_handle);
    registerInterface(&jnt_state_interface_);

    // LEFT_JOINT_NAME: "left_wheel_joint"; RIGHT_JOINT_NAME: "right_wheel_joint"
    hardware_interface::JointHandle left_joint_vel_handle(jnt_state_interface_.getHandle(LEFT_JOINT_NAME),
                                               &cmd[LEFT_JOINT_IDX]);
    vel_jnt_interface_.registerHandle(left_joint_vel_handle);
    hardware_interface::JointHandle right_joint_vel_handle(jnt_state_interface_.getHandle(RIGHT_JOINT_NAME),
                                               &cmd[RIGHT_JOINT_IDX]);
    vel_jnt_interface_.registerHandle(right_joint_vel_handle);
    registerInterface(&vel_jnt_interface_);

DiffDriveController setup:

base_controller:
  type        : "diff_drive_controller/DiffDriveController"
  left_wheel  : 'left_wheel_joint'  # joint 0
  right_wheel : 'right_wheel_joint' # joint 1
  pose_covariance_diagonal : [0.001, 0.001, 1000000.0, 1000000.0, 1000000.0, 1000.0]  # TODO adjust
  twist_covariance_diagonal: [0.001, 0.001, 1000000.0, 1000000.0, 1000000.0, 1000.0]

  wheel_separation : 0.18 # TODO measure in solidworks 
  wheel_radius : 0.03

  linear:
    x:
      has_velocity_limits: true
      max_velocity: 0.5
  angular:
    z:
      has_velocity_limits: true
      max_velocity: 2

  enable_odom_tf: true
  odom_frame_id: odom
  publish_cmd: true