In the unwrap UVW tool, Stitch is used to weld instances, by that I mean the uvs of vertices that are actually welded in the 3d polygon, while weld is used for welding the uvs of any vertices even if they are not welded together in the 3d polygon. Now the question is why would you want to use weld there at all? Does anyone know in what situation you would want to weld the Uvs of an object despite that they are not welded on the object itself? I think there is something I may be missing here.
No. You wouldn't want to weld UV coordinates(UV vertices) together that don't belong together. (UV coordinate in the forehead to a UV coordinate in the forearm) It just wouldn't make any sense. The major difference between Welding and Stitching is that they create the same result but they do it differently. Stitching is kind of like a "Smart Weld Tool". Don't worry, you're on the right track. Hopefully this helps your concern. If anyone else has a comment, please chime in.
-- Did you have this question while watching the UV Mapping Workflows in 3ds Max 2012 course? I'm just curious.
@JoshuaK Thanks for the reply! I actually just recently watched the UV Mapping Workflow course for Max 2012; this was to refresh my memory on how to UVW map in 3dsmax as its been a long time I did that in max. However i had this question when working on the uv mapping of a landscape model I am working on. I was looking for a faster way to stitch uv vertices. I couldn't just select all vertices or segments that belong together and stitch them at once, and even if I did that it would only stitch one at a time, so I tried the weld tool thinking that might do the trick, but soon found out that it was meant for a different purpose. I did a research on it and realise its use. And after that I couldn't help wonder why the tool existed at all in the uv editor interface of the unwrap uvw mapping modifier.