This tool will help you to co-view 2 or more drawings easily.
In Master drawing (or viewports), you type the command then use mouse middle-button to PAN or Mouse-wheel to ZOOM.
All dependent drawings (or viewports) that you choose will automatically PAN/ZOOM the same way keeping the same view, always in real time!
This is particularly helpful when you wish to compare the difference of drawings.
Ex: Floor 1 drawing vs Floor 2 drawing, final drawing vs previous version of itself; etc.