Polybrush: where does it save mesh changes?

Hi, I have a mesh that I'm using as a terrain (not ideal I know, but for implementation reasons I can't use the native terrain) and I'm using Polybrush to adjust some of it. I'm not sure I understand well how it saves changes in Unity. Ideally I'd save it as an asset (and prefab), but for this to work the mesh needs to be a plane, apparently. Does Polybrush save to the scene or elsewhere?

thanks

 

I'm having this problem too, but I can't find this "save to mesh" button anywhere

 

To save to another mesh, go to :
Top Menu > Tools > Polybrush > Bake vertex Streams > Batch option : create a new composite mesh

Then update your prefab.
After that you can update your mesh without create a new one.

That works for me.