I don't think the root object concept applies in all cases. For example, an Advanced Terrain object is split into several sections ("limbs") and limb 0 is the first section.
I suspect it applies to objects that have been created in a modelling package designed for creating a heirarchy of boned limbs. In such cases a root limb might be created by default even if you have only one actual limb or if there is no real heirarchy. If you're working with text based X files you can easily check the contents in a text editor such as WordPad and see the heirarchy yourself - look for sections beginning "Frame" (I think - don't seem to have an example handy).
At least you've got it working.