John asked the following: "a question I have relates to 'vessel not under command'. The lights/day signals to be displayed are different than a vessel at anchor, and I'm trying to figure out what 'not under command' can mean, then? Just floating around? Damaged and not able to be commanded? Just puzzled..."
John is already on the right track. The definition in Rule 3 is: "The term vessel not under command means a vessel which through some exceptional circumstance is unable to manoeuvre as required by these Rules and is therefore unable to keep out of the way of another vessel."
By the way, a vessel that has shut down engines is not necessarily not under command. As long as the crew can restart the engines in time to comply with some obligation that may come up under the COLREGS, it is still under command.
However, the most obvious precursor would be for a vessel to lose power and be drifting. That is usually the first thing that comes to mind for a vessel that falls under the definition. If that were the only circumstance causing NUC, it would be easy to deal with... just avoid the drifting vessel, as you would with a vessel at anchor.
Using the lights or shapes for NUC (instead of "at anchor") allow for many other behaviours. For instance the vessel might have runaway engines and be unable to stop. Or it may have both a runaway engine and stuck steering gear and so be running in circles. Or, due to some peculiar problem with steering gear, the vessel might make unexpected turns. More exotically the crew may be held captive by pirates (off Somalia) and be unable to steer the vessel to comply with the COLREGS... ummm, I guess that is a bit farfetched for operations in the Salish Sea!
The purpose of the NUC category is to emphasize that the vessel may have even more erratic behaviour than if it were simply drifting. So such a vessel needs to be treated with considerably more "respect" than if it were merely drifting or at anchor.
Further comments on this topic will be very welcome.