Software Architect's Handbook

Step 3 – Choosing one or more elements of the system to refine

Based on the iteration goal and the architectural drivers that we want to create a solution for, we must select the various elements that we want to decompose.

If your project is a greenfield system and this is the first iteration, you begin at the highest level and start by decomposing the system itself. For any other iteration, the system has already been decomposed to some degree. You would select one or more of the existing elements to focus on for this iteration.