I never used this attribute. I guess following - the mesh can have up to two uv-maps (multi-texture). So the index is 0 for the first and 1 for the second UV map or -1 for both.
The matrix is applied for all UV-attributes of a UV map. But I have no idea how this is mixed with the “from” uv map. Maybe the “from” map is the source. The matrix is applied to the map and the result goes to the first UV map?
I really do not know.
I had some tests. somehow the “from” argument is not optional. If it is missing I get an error.
If both indices are equal I can apply a 4D transformation matrix (with 2D transformation). The result will be applied to the source/destination map. This is pretty nifty as you do not need to loop though the vertices :).
If I set use transformUV(-1, matrix 0,1) I always get a pink cube ;). This might be a laaaarge scale or just a mistake.
transformUV seems to do nothing.