For what purpose is AFFF used in onboard firefighting?

Aqueous Film Forming Foam (AFFF) is specifically designed for firefighting, particularly in the case of flammable liquid fires, such as those involving fuels and solvents. Its primary purpose is to create a film that suppresses the combustion process by forming a barrier over the burning liquid. This barrier prevents oxygen from reaching the flames, effectively smothering the fire.

Using AFFF is crucial in maritime environments, where the risk of fires involving flammable liquids is significant. Its effectiveness lies in its ability to extinguish fires quickly and efficiently, reducing the risk of fire spread and ensuring safety onboard.

The other options focus on functions unrelated to AFFF’s primary use. For instance, cooling machinery is more about preventing overheating and maintaining operational safety, while trapping heat in spaces does not align with AFFF's role in firefighting. Although cleaning surfaces is an important task in maintenance, AFFF is not formulated for cleaning purposes. Therefore, the focus of AFFF’s application remains strictly in fire suppression, highlighting why it is the correct response to the question.