The Use Existing Surface feature allows manual creation of patching surfaces, or runoff surfaces in other Inventor environments.
The Create Patching Surface and Create Planar Patch features do not always meet your requirements. Examples are when a plastic part has complex holes, or the parting line is unsuitable for a runoff surface.
In those cases, create the patching or runoff surface with the tools in the Mold or Construction environments. When you create the surface manually in another environment, select the existing surface as the patching or runoff surface with Use Existing Surface. You are prompted to select the surface that you created.