In the window, a zoomed in (magnified) part of the picture of the 'Mix Window' is displayed.

This allows to view even big picture accurately, just by moving over its preview image in the 'Mix Window'.

In the 'Zoom X' box, the zoom in value is fixed in the range of 1 - 30 [-/=]. A little cross in the center of 'Zoom' window represents the current position of mouse cursor. Window title bar displays exact positon. Additionally the information includes which source is currently zoomed and if the zoom is locked or not. The bottom panel shows current color under the mouse cursor and their RGB values. Image in the 'Zoom' window is refreshed just as the cursor is shifted in the 'Mix Window' preview area. You can lock zoomed area by right-clicking in the 'Mix Window'. If zoom is locked and video source is previewing, image in the window is refreshing automatically.
Somewhat different solution is in the digital still camera and 'Folder Monitor' mode. When stored-live mix is set to 100%, zoom shows image from the Preview window, below this value zoom shows current frame of the sequence.
Zoom tool may be useful i.e. for precise setting of markers (video) or checking out the focus (digital still camera).

If the Live View is enabled (capture device is set to 'EVF Live View') and display for video is set to 100%, in the Zoom window native EVF image appears.