Make sure the object has UVs and a
Material assigned to it.
You also need to have a texture projected onto a model in order to use the UV Fuse tool. This tool is useful for merging UVs once they are unwrapped.
Using UV Fuse
Click the
UV Fuse button on the Texture tab.
Select the UVs you want to fuse and press Enter to confirm your selection.

For specific parameter help see the
UV Fuse node help.