FAR part 91 Sec 91.117 for example states that "unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots". All of the major jurisdictions I can find reference to have the same rule.
I've highlighted MSL (mean sea level). Why is MSL used? What's the point of such a regulation when aircraft would be able to manoeuvre at high speed close to the surface of high elevation fields?
How, if at all, is this varied when operating to or from an airfield with a high elevation, for example Quito, Ecuador (SEQM) at 7,910 feet?