Which of the following best describes the term 'ullage'?

The term 'ullage' specifically refers to the portion of a tank or container that is not filled with liquid, effectively describing the empty space that remains above the liquid level. This measurement is crucial in various contexts, such as understanding the storage capacity of tanks and ensuring safety during loading and unloading operations.

When transporting liquids, maintaining appropriate ullage is important to prevent overflow and accommodate for thermal expansion of the liquid, which can lead to spillage and safety hazards. The ability to accurately assess ullage directly impacts efficient cargo management and regulatory compliance.

In contrast, the other options have distinct definitions that do not align with the meaning of ullage. The volume occupied by the cargo pertains to the total liquid present in the tank, while the measured distance from the bottom of the tank to the liquid surface is known as 'liquid level,' and the total capacity of a tank refers to how much it can hold when full, not the empty portion. Therefore, 'ullage' captures the essence of space that remains unfilled within the tank.