Screens are mostly assembled from screen sections (screen section instance is a link to a Screen Section Definition so you can reuse screen sections in multiple screens). To make the user interface more intuitive and readable for the user, additional widgets like Tab Control or Split Panel can be used.
If you are using a package which has a screen defined and you want for example, add a new tab to the screen with your own customized section or you want to provide your own screen section instead of the provided one you can do so by using Alternatives.
You can create a screen alternative by right clicking on your screen element and selecting New > Alternative. A new version of the original screen will be created in your own package. This version will be used instead of the original version. You can make as many edits to it as you want. Obviously if the original screen is updated (new widgets added) your version will stay intact (you will need to add those widgets manually for the screen alternative you created).