
First of all, it brings you possibility to set the fallback texture to your eye. Basically, this texture will be used with the default unlit shader on your eye ("_MainTex") if someone blocks your avatar shaders in VRChat.
Just apply your eye material to a quad and screenshot it. I think, that's the easiest way.
To check how it works on your avatar, change the shader from Universal Eye Shader to unity's default Unlit Texture shader (don't forget to change it back after that).

Now you're able to resize and move the eye across the UV in case if you have a custom UV mapping of your eye.
>>> Read more about the Universal Eye Shader <<<
>>> Download the Universal Eye Shader <<<