Usually this output device is the computer screen or a loud speaker, but can be any other device which makes information accessible to humans. Objects may have more than one representation. See Section 3.3 for further discussion. Actors are the basic object type. An actor might be either a simple physical entity such as a doctor, a dog, a bicycle or tree a more complex physical entity, e.g. city, forest, people an immaterial entity, e.g. ideas or concepts such as politics an adjective, e.g. beautiful, old, tall, nice a verb or description of doing or going on, e.g. talking, running, hugging Actor objects have representations of their own.
Compound objects are basically groups of objects.
Compound objects have no representations themselves. Realizing compound objects is done by realizing the objects they consist of. See further discussion in Section 4.1 . Relations are objects which establish a relationship between objects. A relation contains a set of describing objects.