Which vessel type is not under command in the collision hierarchy?

In the context of the COLREGs, a vessel "not under command" refers specifically to a situation where a vessel is unable to maneuver as required by the Rules due to exceptional circumstances. This typically applies to vessels that are experiencing mechanical failure or other serious difficulties.

The term "restricted in her ability to maneuver" is distinct, and it applies to vessels that, due to the nature of their work or the situation they are in, cannot take the necessary actions to avoid collisions. Examples include vessels engaged in underwater operations, dredging, or servicing of aids to navigation. These vessels have a defined status within the hierarchy of vessel right-of-way, meaning they have specific navigational responsibilities and privileges.

The other vessel types mentioned—power vessels, sailing vessels, and fishing vessels—are categorized differently under the COLREGs and do not fall under the "not under command" classification. Thus, the answer clearly outlines that "restricted in her ability to maneuver" is the term that does not fall under the category of a vessel that is not under command in the context of collision avoidance and hierarchy.