I believe the demarcation is 400 ft
"Minimum safe altitude" (assuming no taller obstructions are nearby), or something like that. It seems that applies to "navigable airspace", referring primarily to manned aircraft but now extending to the larger UAVs.
Smaller uncertified R/C aircraft may be flown below 400', but there you start getting into the issues of warrants and illegal recordings - and property rights (some of which *do* extend up to the aforementioned minimum safe altitude).
Seems like the R/C forums would be the place to look for the details on this stuff.