Urho docs say this about it Material.SetGlobalVar:
Set global variable with the respective key and value.
What does this mean? It sounds potentially appealing. I often have many copies of the same material, and I have need for updating the same parameter on all of them to the SAME VALUE. Currently, I’m doing a foreach() through all material instances and called “SetShaderParameter(…)” for each. It would be nice to have a global variable that they all look to for their uniform value.