Light or sound signals that cannot be mistaken for any other signal.
Rule 36: Attracting attention
If necessary to attract the attention of another vessel, any vessel may make light or sound signals that cannot be mistaken for any signal authorized elsewhere in these Rules, or may direct the beam of her searchlight in the direction of the danger, in such a way as not to embarrass any vessel. Any light to attract attention shall be such that it cannot be mistaken for any aid to navigation, and shall avoid the use of high-intensity intermittent or revolving lights such as strobe lights.
Wording follows the International Regulations as printed in the US Coast Guard Navigation Rules handbook (2024), with British spelling, and is shortened in places. Download the rules
Read the whole of Rule 36 on one page
Rule 36: Attracting attention
If necessary to attract the attention of another vessel, any vessel may make light or sound signals that cannot be mistaken for any signal authorized elsewhere in these Rules, or may direct the beam of her searchlight in the direction of the danger, in such a way as not to embarrass any vessel. Any light to attract attention shall be such that it cannot be mistaken for any aid to navigation, and shall avoid the use of high-intensity intermittent or revolving lights such as strobe lights.
A training aid, not the official text. The COLREGs are an IMO convention, and your national authority publishes the version that applies in its waters. Check the rules themselves before you rely on them at sea. Download the rules.