Final answer:
The distress signal is used to indicate a vessel in distress and in need of assistance.
Step-by-step explanation:
A signal to indicate a vessel is in distress and requires assistance is the distress signal. The most commonly used and internationally recognized distress signal is the International Morse Code distress signal, which consists of three short signals followed by three long signals and then three short signals again.
This signal can be sent using a radio, light, sound, or any other appropriate means of communication on board the vessel. It is important for vessels in distress to use this signal to attract attention and indicate their need for assistance.