Screen fields are used for output of pictures and videos. When creating DVD/CD menus, they can also stand for buttons for menu operation. How to open the Screen Field Properties:
In
the Media Pool click on the plus sign in front of category 
Screens and select the corresponding
screen following which the screen fields will appear on the right.
Right-click the corresponding screen field and select Properties. This dialog allows the following settings:
Under Aspect ratio you can choose whether the aspect ratio of pictures or videos is to correspond to the pixel values or whether they are to be stretched to screen field size. Note: Distortion correction of videos is normally effected by entering the correct aspect ratio. See Video Properties.
Keep aspect ratio of picture/video ...causes images and videos to be displayed without distortion so that the screen field may not be filled entirely. When the image aspect ratio of picture/video deviates from the screen field aspect ratio, transparent areas will become visible on the right and left edge or at the top and the bottom.
Always use aspect ratio of screen field for picture/video ...ensures that all pictures and videos fill the screen field entirely and may be distorted.
Use aspect ratio of screen field for picture/video if difference to screen field is less than ...causes pictures and videos to fill the screen field entirely and may cause a distortion if the specified tolerance is exceeded. This tolerance value is the factor that decides whether format-filling display is to be used or not. This is a very useful option to avoid transparent areas when the aspect ratio differs only slightly due to scanning errors or the aspect ratio of digital cameras differs only marginally.
Here you can define the way the picture is to be displayed in the screen field. All these options do not cause any distortion of picture or video.
Fit in screen field ... the picture or video is fully visible in the screen field. When the image aspect ratio of picture/video deviates from the screen field aspect ratio, transparent areas will become visible on the right and left edge or at the top and the bottom.
Fill screen field ...the picture or video is enlarged to such an extent that it fills the screen field entirely. For this purpose a corresponding control track is automatically created for the object. Note: If you press Ctrl + Alt when dropping the picture or video in the timeline you can use this type of display temporarily, i.e. without screen field option.
Keep original size ...causes native display of pictures and videos, i.e. 100% pixel accurate. The currently set screen output size serves as a reference. See also Screen Field Properties and Screen Properties. Note: If you press Alt when dropping the picture or video in the timeline you can use this type of display temporarily, i.e. without screen field option.
See also