The flexibility of URDF reduces when we work with complex robot models. Some of the main features that URDF is missing are the simplicity, reusability, modularity, and programmability.

If someone wants to reuse a URDF block ten times in his robot description, he can copy and paste the block ten times. If there is an option to use this code block and make multiple copies with different settings, it will be very useful while creating the robot description.

The URDF is single file and we can't include other URDF files inside it. This reduces the modular nature of the code. All code should be in a single file, which reduces the code simplicity too.

Also, if there is some programmability, such as adding variable, constants, mathematical expressions, conditional statement, and so on, in the description language, it will be more user friendly.

The robot modeling using xacro meets all these conditions and some of the main features of xacro are as follows:

• Simplify URDF: Xacro is the cleaned up version of URDF. What it does is,
  it creates macros inside the robot description and reuses the macros. This can reduce the code length. Also, it can include macros from other files and make the code more readable, simpler, and modular.
• Programmability: The xacro language support a simple programming statement in its description. There are variables, constants, mathematical expressions, conditional statements, and so on that make the description more intelligent and efficient.

We can say that xacro is an updated version of URDF, and we can convert the xacro definition to URDF whenever it is necessary, using some ROS tools.

We can discuss the same description of pan and tilt using xacro. Navigate to chapter_2_code/mastering_ros_robot_description_pkg/urdf, and the file name is pan_tilt.xacro. Instead of .urdf, we need to use .xacro for the xacro file definition. Here is the explanation of the xacro code:

<?xml version="1.0"?> 
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="pan_tilt"> 

These lines specify a namespace that are needed in all xacro files for parsing the xacro file. After specifying the namespace, we need to add the name of the xacro file.