ROS2 package.xml explanation

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What is the purpose of following snippets in package.xml

1)

   <export>
    <build_type>ament_cmake</build_type>
  </export>

2)

<member_of_group>rosidl_interface_packages</member_of_group>

3)

<build_depend>rosidl_default_generators</build_depend>
 <exec_depend>rosidl_default_runtime</exec_depend>

4)

  <depend>rosidl_default_generators</depend>

: when to use and not to use 1,2

: Are 3 and 4 are equal?

I looked up the https://ros.org/reps/rep-0149.html but couldn't figure it out.

Thanks for help.