Undock vs decouple node for docking ports

[THX1138]

What is the difference between "decouple node" and "undock" in the menus for the Clamp-O-Tron docking ports? (see menus in screenshot below). Decouple node ejects the whole piece regardless of the lower docking port whilst undock does not affect the connection with the upper docking port.

 

 

[Reactordrone]

Only one of those docking ports will have been attached in the VAB due to the tree structure building model in KSP. That one will be giving you a decouple node option. I assume the bottom end is trying to dock but isn't actually attached and when you kick off the top the unit falls before the magnetic attraction can dock the lower port properly.

[Snark]

Just to be clear:  There are two ways that a thing can be attached to a docking port:  docked or coupled.

A docked thing is something that is attached "in flight" via docking.  The only possible part here is another docking port of the correct size.

A coupled thing is something that you attach to the docking port in the vehicle editor before launch (e.g. in the VAB).  Unlike docking, this can be anything with a stack node, not just a docking port.  For example, if you've got a docking port on the top end of your rocket, you can stick a nosecone on top of it for aerodynamics during ascent.  You don't need to add a stack decoupler-- you can just attach the nosecone directly to the docking port.

"Undock node" releases a docking connection.

"Decouple node" releases a coupled connection.