The Render Iterative command, available from the Render flyout on the main toolbar, renders the scene in iterative mode without opening the Render Setup dialog.
Iterative rendering ignores file output, network rendering, rendering of multiple frames, and email notification. Use this option when doing quick iterations on the image, usually in parts; for example, working on reflections, specific objects, or areas of the scene.
Also, when rendering in Iterative mode, rendering Selected or Region leaves the rest of the Rendered Frame Window intact.
Tip: To activate Iterative rendering mode without launching a rendering, use the Target drop-down list on the Render Setup dialog, or the drop-down list at the top-right corner of the Rendered Frame Window.