The Viewport ROP starts a 3Delight IPR render directly into the viewport, à la Solaris. The Viewport ROP icon in the toolbar is a quick and easy way so start and stop such an IPR render. One can also create the ROP manually and start the render from there.
When using the free licensing, the viewport IPR render will consume the license so no other parallel renders are possible. Make sure to stop the render before starting another one. The 3Delight Pro license doesn't have this limitation.
Note that this ROP has only the quality and the override parameters. Other parameters found in the normal ROP are not needed in such a context.