Screen Write is similar to screenshot feature. When you choose it, the app captures the image of active screen and then give you a editor through which you can right on it. It nice to mark some important information for future reference.
Step 1: Enable Air Command from Settings > Controls > Motion Control > Air Command.
Step 2: Hover S-Pen over Screen by pressing S-Pen Button to launch Air Command.
Step 3: Select Screen Write from Air Command Menu
Step 4: The app will capture image of live screen. Using the S-Pen write anything on it and share or save it.