Which light characteristic may be used on a special purpose mark?

For a special purpose mark, the light characteristic that may be used is a fixed light. Fixed lights provide a constant light, which is often used to indicate certain types of significance or to highlight locations where ongoing attention is needed. Special purpose marks are used to designate certain areas or aids that do not fall under traditional navigational needs but still require visual identification.

Other light characteristics, such as occulting, equal interval, and quick flashing, may serve different purposes and are not as commonly aligned with the function of special purpose marks. For instance, occulting lights provide a pattern of light and dark that is typically associated with buoys marking a specific hazard or feature, which does not align with the idea of a constant marker for special purposes. Equal interval and quick flashing light characteristics could be more suitable for navigational aids rather than the more specific identification requirements that special purpose marks entail. Thus, the fixed light characteristic is best suited for conveying the nature of a special purpose mark effectively.