Author here, great question :) If principles are the elements, we can think of each system as a "molecule" with some imagination. For example, an SQL database system has many principles:

1. Abstraction Lifting (Al) + Policy/Mechanism Separation (Pm): SQL states high-level intent with precise semantics, and logical operators are decoupled from physical operators.

2. Equivalence-based Planning (Ep) + Invariant-Guided Transformation (Ig): We apply algebraic rewrites that preserve semantics (e.g., join reordering, predicate pushdown) under stated invariants.

3. Cost-based Planning (Cm): We choose concrete physical operators and join orders using a cost model and so on..