Texture Baking View lets you preview render a specific portion of your scene so that you can get a feel for the changes you make as you shade, light, and texture objects.
Unlike rendering at a lower resolution, a process which reduces the size of the entire image, previewing a region can help you make changes more efficiently and quickly to the specific regions of interest at full resolution.
If you turn on Auto Preview Region (Options > Auto Preview Region), the changes you make appear as you draw the marquee anywhere in Texture Baking View. Only the marqueed area of the surface re-renders showing the results of the adjustment.
To preview a region of your scene