About the definition, we could do a quick survey but I guess that the definition adopted by OpenFlight, OpenSceneGraph and X3D probably can be merged into a single one as they rely on the same idea. Which is: a LOD is a group node, its children are the various representations. The LOD node has a list of distances at which the switch from one level to the next one is supposed to happen. That's it. We can then decide the details of how to store the ranges but those 3 representations are basically equivalent. And for example Maya has exactly the same idea so exporting it would be straightforward (I've done it already for the OSG exporter by the way).