When using RDI Acquisition Software, if you left-click and drag the cursor on the screen, you will draw a red box. This box becomes the "Region of Interest". The image will be cropped to include ONLY the video inside of this box. All surrounding images are ignored. You can use this to reduce the amount of data recorded and stored by the camera. This becomes useful as you try to record at higher Frames per Second or Fmax values.
Using the ZOOM icons simply allows the user to focus on a smaller portion of the video being recorded. This is useful for improving the focus of the image. The difference in this case, is that the image surrounding the current view is still recorded.
See below for examples (assume the asset in these pictures is always the same size):