The default rendering procedure is to render all objects in the current view of a drawing. While the rendering process is faster when you render an area of a view, rendering the entire view lets you see how all objects are oriented.
If your drawing contains named or camera view, you can quickly restore a view with the View Controls in the upper-left corner of the current viewport or the VIEW command.
Sometimes you need to render only a portion of what is displayed in a viewport, but you still want to see some of the surrounding environment. By using the Render Region tool, you can specify a rectangular area in the current viewport to be rendered. Any objects that appear in the area are rendered. Everything outside the region is ignored by the renderer.