ROS2 xacro parsing: ${robot_ip} vs $(arg robot_ip)

edit

Recently, I found this does not parse: $(arg robot_ip)

Whereas this does parse: ${robot_ip}

Do they have different purposes? And to make things stranger, both varieties seemed to work on my coworker's computer.

Here's the full block of code, if it would be helpful:

  <xacro:macro name="load_arm" params="
    parent
    dof
    vision
    prefix
    *origin
    robot_ip
    use_fake_hardware:=false
    fake_sensor_commands:=false">

      <!-- ros2 control include -->
      <xacro:include filename="$(find my_robot_description)/arms/gen3/${dof}dof/urdf/my_robot.ros2_control.xacro" />
      <xacro:my_robot_ros2_control
        name="MyRobotMultiInterfaceHardware" prefix="${prefix}"
        use_fake_hardware="${use_fake_hardware}"
        fake_sensor_commands="${fake_sensor_commands}"
        tf_prefix=""
        robot_ip="${robot_ip}" />