What's the differences between the packages openni_camera and openni_launch?

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There are 2 launch files kinect_frames.launch and openni_node.launch and 1 node openni_node in the openni_camera package.

And for openi_launch package, there are several launch files

depth.launch             device.launch            kinect_frames.launch     rgb.launch               
depth_registered.launch  ir.launch                openni.launch

and no nodes.

I'm wondering the differences between openni_camera/openni_node.launch and openni_launch/openni.launch.

And what are the other launch files such as depth.launch and device.launch in openni_launch package for? How do I use them?

Thanks for answering.