An ABSE model, like in other MDSD methodologies, is abstracted at multiple levels:
Only one Atom exists: It is ABSE's meta-metamodel, the model that describes how to build models of your model...
Confused? Let's see it in another way:
When you want to build your application, you first build a model of it, right?
But to build a model of your application, you first need to know how to build models.
And to build coherent models, you have to follow a unified mechanism. For that, you need a model too!
So, the Atom is the unified mechanism to build models.
Atom Templates define the rules of how to build a model of an application and its components.
Atom Instances implement your model.