GKS provides a way to collect and store together the primitives that make up all or part of a picture. Such a collection of primitives is called a segment, has a unique name, and may be manipulated as a unit in various ways. Only one segment can be open at a time, and once a segment has been closed further output primitives cannot be added to it, nor can the primitives in it be modified.
Any non-retained data output whilst there is no open segment will be lost if the screen is cleared for some reason. There are several circumstances when this may be useful. For example, a message written on the screen may have only limited validity, or one may wish to output a very complicated image with too much data to be stored, or which needs only to be displayed once. However, much of the power of GKS comes from the ability to interactively modify the image without always having to regenerate the complete picture. To make use of this feature, primitives must be stored in segments.