As I understood from the specification, all vertex data inside <polygons>, except position is referred to the <source> tag. The vertex position is special and the <source> is referenced indirectly via the <vertices> tag.
In one of the example files (collada.dae) vertex position in referred directly to the <source> in one of the <polygons>. Is that correct?
Why vertex position has a dedicated tag, unlike all other parametres? According to the Core Categories in the chapter 2 (Format Overview), using dedicated <vertices> tag has a subtle advantage. Then why the rest of the parametres do not have their own tags: <texcoords>, <normals>, etc? That could make it consistent with vertices and would make possible to use the same code instead of making a special case for vertices' positions.