These can be handled as standard messages from the graphics device to the serving host. The second class of interactions is with the picture itself. This is more difficult because of the problem of associating the point selected with some meaningful entity such as a list. This association can probably only be made by the serving host since the using host, or a Form Machine, may have transformed the NGDS in unknown ways, and the NGDS may no longer exist at the using host's site. There are essentially two classes of devices that can interact with the picture.
Light pens are synchronous devices.
Asynchronous devices are those which can interact regardless of which, or even when no, display item is being executed. Asynchronous devices make associations more difficult since no relationship can be easily inferred as they can for synchronous devices. The NGES is created for the interaction with the picture. For synchronous devices the current values for the list name, origin, and NGLP, as well as the contents of the NGES provides a hierarchical structure where associations can be made. For asynchronous devices, the structure is probably not as necessary as the name and coordinates within the image space in which the interaction occurred.