"stopping" state vs documented "shutting_down" state?

[Koper, Dies]

I was looking at the instance states and noticed rhevm, ec2 and
opennebula have a state ":stopping":

    running.to( :stopping )       .on( :stop )

This state is not documented in 3.4 Instance States of the API
documentation.
What is the difference between this state and shutting_down?

Thanks,
Dies