Rule 3
General definitions
The vocabulary the rules are written in: underway, not under command, restricted in ability to maneuver, in sight, restricted visibility.
In plain language
Every word in the rules is defined precisely, and exams love the traps. “Underway” just means not anchored, not moored, and not aground — a boat drifting with her engine off is still underway. “Not under command” means some exceptional circumstance (like a failed engine or steering) prevents maneuvering — not that nobody is aboard. A “sailing vessel” stops being one the moment her engine is in gear, and a vessel “engaged in fishing” means gear that restricts maneuverability — trolling lines don't count.
This rule differs under the US Inland Rules. Change the rule set in language and regional settings to see the difference.