A more thorough explanation:

Definition: A customs frontier is the point at which a country imposes customs duties on goods entering its territory. It is the territorial boundary where customs officials check and collect taxes on imported goods.

Example: If a truck carrying goods crosses the border from Mexico into the United States, it will pass through a customs frontier. At this point, U.S. customs officials will inspect the goods and collect any applicable taxes or fees before allowing the truck to continue on its journey.

Explanation: The example illustrates how a customs frontier works in practice. When goods cross a border, they must pass through customs, where officials check to make sure they comply with all relevant laws and regulations. If the goods are subject to taxes or fees, these will be collected at the customs frontier before the goods are allowed to enter the country. This helps to ensure that countries can control the flow of goods across their borders and collect revenue from imports.