The Reference Monitor acts like a secondary Program Monitor. You can use a Reference Monitor to compare different frames of a sequence side by side, or to view the same frame of a sequence using different viewing modes.
You can cue the frame of a sequence displayed in the Reference Monitor independently from the Program Monitor. This way, you can cue each view to a different frame for comparison—to use the color matching filter, for example.
Alternatively, you can gang the Reference Monitor and Program Monitor together, so that they both show the same frame of a sequence and move in tandem. This is especially useful for color-correcting tasks. By setting the Reference Monitor’s viewing mode to a waveform monitor or vectorscope, you can make adjust the color corrector or any other video filter more effectively.
You can specify the Reference Monitor’s quality setting, magnification, and viewing mode just as you would in the Program Monitor. Its time ruler and viewing area bar also work in the the same way. But because it’s for your reference and not for editing itself, the Reference Monitor contains controls for cueing to frames and not for playback or editing. When you group the Reference Monitor and Program Monitor together, you can use the Program Monitor’s playback controls. You can open only one Reference Monitor.
Andrew Devis shows how to correct brightness and contrast, and use of the Reference Monitor in this video on the Creative COW website.
You can gang the Reference Monitor and the Program Monitor so that both always monitor the same frame.