Are there any fixed principles
for subdividing the WBS?
Not really. Some “experts” will argue that the WBS must reflect a product decomposition, but this position is not widespread outside of systems engineering or universally accepted within systems engineering. Your guiding principle: decomposition principles should reflect the way the team wants to manage and control your project. Sometimes this is involves product decomposition. Many times it is chronological, or by functional departments, geography, cost accounts, and often it is a combination of these.

In general, there is no single best way to organize the WBS. Any given project can be decomposed several ways. Your task is to find the decomposition that works best for the team. We strongly suggest developing at least two different Work Breakdown Structures during planning to examine different management approaches and to compare the benefits and restrictions of each. Then, chose the one that best reflects how the team wants to control the project.