A com.nomagic.magicdraw.uml.symbols.PresentationElementRendererProvider is an object, which provides a custom symbol renderer for a given presentation element.
PresentationElementRendererProvider.getRenderer(PresentationElement) method provides a specific renderer for the given com.nomagic.magicdraw.uml.symbols.PresentationElement (Symbol) or null.
To make your provider start working, you must register it into the com.nomagic.magicdraw.uml.symbols.PresentationElementRendererManager using:
PresentationElementRendererManager.getInstance().addProvider(new RendererProvider());