An atom that represents a particular concept is called a model . It's called a model because it models that concept. The above data structures, though, don't seem to have anything to do with concepts such as integers or programs. They're extremely low level, and in fact, they don't really seem to have any meaning. The lack of meaning is intentional.
Prism is a general-purpose tool, which means that it can support any programming concept.
But for it do so, it can't make any assumptions about what sort of concepts you'll need. Therefore, you're expected to create your own high-level concepts. You do so by combining the data structures together to create larger structures and then defining what concept they represent. These larger structures and their descriptions are called metamodels . In the programming world, a metamodel is a model that represents a set of models.