Resume Rendering lets you continue a rendering which has been completed - either because the number of passes or rendering time limit has been exceeded, or because you clicked the Stop Rendering button.
IRender nXt improves its image with each rendering pass. Resume Rendering lets you render for more passes without having to restart the rendering from the beginning.
You can enter more passes, or more time to let it render longer. (Enter 0 for no limit)
Double Up - click this button to double the current settings.
Note: If the rendering was not stopped properly - at the end of a pass - then the Resume Rendering icon will be grayed out.