Well, they are indeed indices. But it's part of a bigger whole. I just exported a simple box with normals and mapping coordinates. First you get three float_array elements with the positions, normals and mapping coordinates grouped in sets of 3, 3 and 2. Here is part of the position array:

<float_array id="geom-Box001-positions-array" count="24">-9.624062 -10.42627 0 9.624062 -10.42627 0 -9.624062 10.42627 0 9.624062...

The first xyz position with index 0 is -9.624062, -10.42627, 0.

Then you get the triangle indices. Each triangle of course consists of the three vertices. Each vertex has three indices. One into the position array, one into the normal array and one into the mapping coordinate array. Here is part of the triangle indices:

<triangles material="ColorMaterial" count="12">

<input semantic="VERTEX" source="#geom-Box001-vertices" offset="0"/>

<input semantic="NORMAL" source="#geom-Box001-normals" offset="1"/>

<input semantic="TEXCOORD" source="#geom-Box001-map1" offset="2" set="0"/>

<p>0 0 9 2 1 11 3 2 10 3 2 10 1 3 8 0 0 9...

</triangles>

The first triangle is indexed as 0 0 9 2 1 11 3 2 10. The first vertex of the first triangle is indexed as 0 0 9.

It has position index 0, normal index 0 and mapping coordinate index 9.

This is an actual example of course. Since you are the one to generate the file, you can be a bit more logical than 0, 0, 9.