Please tell me the meaning of the description in the launch file

edit

I couldn't understand the meaning of the following description(value) in the launch file. I couldn't understand a description that combines eval, if and else.

<arg name="uf_traj_controller" value="$(eval 'lite6_traj_controller_velocity' if arg('velocity_control') else 'lite6_traj_controller')" />

Please tell me the meaning of description of value.

A part of launch file

<launch>   
  <arg name="run_demo_traj" default="false"/>
  <arg name="effort_control" default="false"/>
  <arg name="velocity_control" default="false"/>
  <arg name="namespace" default="ufactory"/>
  <arg name="add_gripper" default="false" />

  <!-- Load joint controller configurations from YAML file to parameter server -->
  <!-- Load trajectory controller for xArm by default -->
  <rosparam unless="$(arg run_demo_traj)" file="$(find xarm_controller)/config/lite6_traj_controller.yaml" command="load" ns="$(arg namespace)"/>
  <arg name="uf_traj_controller" value="$(eval 'lite6_traj_controller_velocity' if arg('velocity_control') else 'lite6_traj_controller')" />
  <node unless="$(arg run_demo_traj)"
    name="traj_controller_spawner"
    pkg="controller_manager"
    type="controller_manager"
    respawn="false"
    output="screen"
    ns="/$(arg namespace)"
    args="spawn $(arg uf_traj_controller) joint_state_controller"/>

  <!-- run_demo: load the position controllers -->
  <rosparam if="$(arg run_demo_traj)"
    file="$(find xarm_controller)/config/lite6_position_controllers.yaml" command="load" ns="$(arg namespace)"/>
  <node if="$(arg run_demo_traj)" name="position_controller_spawner"
    pkg="controller_manager"
    type="spawner"
    respawn="false"
    output="screen"
    ns="/$(arg namespace)"
    args="joint1_position_controller joint2_position_controller
          joint3_position_controller joint4_position_controller
          joint5_position_controller joint6_position_controller
          joint_state_controller"/>


</launch>