Different categories and decisions we make as we go through the design process.
Instructional Strategy (constructivist, behaviorist, positivist)
Representation of the Message
Specialized language based on the discipline (computer vs print, vs class)
Any decision made on one layer has a cascading effect on the others. Often times constraints will impact some of these layers and therefore all the other layers.
Each decision you make collapses the total number of possible decisions that you are capable of making.
Design is full of decisions. Layers is one way to look at the major decisions that need to be made in design and how they impact each other.