There's a business arrangement called the "Atlantic Plus Plus Joint Venture" via which a bunch of carriers share each others' revenues for travel across the ocean.

Those carriers include a bunch of Star Alliance carriers who operate transatlantic flights, including United, Lufthansa, Swiss, Austrian, Air Canada, and Brussels Airlines.

These carriers tend to set identical prices & fees, including a "YQ" fee, sometimes called a "fuel surcharge", which adds to the bottom-line price of a ticket and is set by the carrier based on origin & destination city pair.

When someone says "$516 appears to be the current fuel surcharge for roundtrip coach across the A++ JV", what they mean is that "most carriers who participate in the Atlantic Plus Plus joint venture have agreed to charge a $516 fuel surcharge for a coach-class round trip flight across the Atlantic ocean", i.e. this is one of the fees that most of those carriers who sell transatlantic tickets will all charge.

Carriers separate out YQ and base fares for international travel for, I have to assume, marketing purposes (they can advertise low base fares but they have also baked in a high fixed fare component in one of the surcharges which they take as revenue).

So, to answer the question directly, an "international surcharge" is part of the price of an airline ticket that some airlines choose to charge as a surcharge rather than as base fare. In the US, laws usually require that airlines show all-included prices inclusive of base fare & surcharges, so this arrangement doesn't help the airlines get away with anything sneaky; in other places, they may be able to advertise a $50 fare with a $516 pure-profit surcharge. In practice, this means that the lowest price you can *ever* pay for a ticket on a carrier between a certain pair of cities is bounded by the fuel surcharge they publish (which they change occasionally, but not as often as they change fares).