The use of the term differs in Europe, the octane advertised at the pump is the RON number (which is always higher than the MON number).
In the U.S. we use the average of the two number often referred to as R+M/2 .
For example a fuel that has a RON of 110 and a Mon of 87 would be sold as 110 octane fuel in Europe, and would be sold as 98 or 99 octane here in the States.
That is for commercial sales at the pump. Many racing fuels specify which octane number they are reporting.
The MON number is usually more important for racing applications especially turbocharged and blown because that octane number uses a more sever test condition with higher intake air temperatures than the RON test does.
Larry