A vessel not under command is one that has suffered a steering failure or a loss of propulsion power. The term "vessel restricted in her ability to maneuver" is applicable to one attached to the bottom for work, such as a dredge, or a Coast Guard buoy tender servicing an aid to navigation, or a ship laying underwater cables. A tug towing a barge is not normally considered to be restricted in her ability to maneuver, but there may be situations where this is warranted and additional lights and shapes may be displayed.
A vessel engaged in diving operations may or may not be restricted in her ability to maneuver. A salvage vessel with "hard hat" divers down - supplied with air from pumps on the surface, is definitely restricted in her ability to maneuver and should show the lights of Rule 27(d) or (e). On the other hand, a dive boat with free-swimming SCUBA divers in the water is definitely not restricted and should not show the signals of Rule 27(e). Such a craft should fly the "diver's flag" (red with one white diagonal stripe) that signifies "Divers in the water, keep clear." This flag is not a part of the Navigation Rules, but is used and is legally required by many state and local authorities. Rule 27(e) requires "a rigid replica" of the International Code flag "A".