I am loading a COLLADA model. Although personally I didn't think it has that much to do with the format specifics. But I may be wrong.....
Collada contains a list of indices into each of the vertex attributes, so yep, it absolutely does have to do with the format specifics
The index stream will say something like 1 2 3 4 5 6
That means build a vertex using position #1 and UV #2, then build a vertex using position #3 and UV #4, etc... (assuming the mesh only has positions and UV, and that positions are the first attribute and UV's are the second attribute).
Check out the spec.
What particular kind of COLLADA tag are you trying to parse?