Which of the following best describes the term 'distress signal'?

The term 'distress signal' best refers to a signal indicating that assistance is needed. This concept is crucial in maritime operations, as distress signals are specifically designed to alert nearby vessels and rescue services that a vessel or person is in danger and requiring immediate help. The effectiveness of these signals is vital for initiating the appropriate rescues in emergency situations, ensuring safety at sea.

The other choices don't align with the definition of a distress signal. Routine messages between vessels pertain to standard communication protocols, while formal communications with regulatory authorities involve different procedures entirely. Notifications of maintenance work do not indicate an emergency situation, so they do not resonate with the urgency implied by distress signals. Thus, the correct answer captures the essence of what a distress signal truly represents in maritime safety.