I would think a maximum specified speed would be attainable speed in level flight. A “structurally do not exceed speed” is usually identified as such. Then you have the “upside down” situation where the plane is capable, in level flight, of exceeding it’s “structurally do not exceed speed.” This is very often the case in FS, at least, usually with civil airliners, both prop-liners and modern jet liners. Especially the modern jet liners. I wonder how true this particular situation really is in real life. The FS 747-400 will “explode” if you exceed mach 0.92, which is easily done at way less than full power, at any altitude, so you have to watch your speed very carefully. But Fliger747 says he can fly his 747 very close to mach 1.0. I wonder if there are any real planes that can exceed their “do not exceed” speed in level flight…