Final answer:
The US requirement for Vietnamese catfish to be labeled differently is an example of a non-tariff barrier, a method used by governments to regulate imports without tariffs through specific rules and regulations.
Step-by-step explanation:
The requirement by the US government for imported catfish varieties from Vietnam to be labeled as tra, basa, pangasius is an example of a non-tariff barrier. Such regulations are ways nations can control imports without imposing tariffs but by drawing up specific rules, regulations, inspections, and paperwork. This particular case exemplifies a non-tariff barrier in the form of labeling requirements, which aim to distinguish domestically produced goods from imported ones and potentially influence consumer choice, protecting domestic industries.