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Difference of ros2 demos and ros2 examples

as the question has been unanswered on answers.ros.org I hope here someone kindly comments

Tutorials refer to ros2 demos and to ros2 examples and both are actively maintained
both seem to rely on example_interfaces in /opt/ros/crystal/share/example_interfaces

Is that a historically grown thing ? if not when should I to look at which ?
Or in other words how would I extract the right files/infos to compose my own ros2 project ?

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