Final answer:
CAO (Cargo Aircraft Only) is a handling label used in air transport to signify that a package containing hazardous material should not be carried on passenger aircraft for safety reasons.
Step-by-step explanation:
The term CAO (Cargo Aircraft Only) refers to a handling label that is used in the transportation of hazardous materials by air. This label indicates that the package can only be transported on an aircraft that is designated to carry cargo, not on passenger aircraft. This is critical for ensuring safety as certain hazardous materials are not permitted on aircraft with passengers due to the potential risks they pose. The CAO label is one of many important markers in the regulation of air transport for goods that might pose a danger to people, property, or the environment.