A.19.SelectorMechanism:10 - Rationale

Selection is where many systems accidentally convert admissible but nuanced information into an unjustified scalar decision. Making selection a separate, explicit mechanism boundary achieves two things that matter for engineering management:

1. Technical integrity: it enforces admissibility and evidence discipline at the decision boundary without smuggling heuristics.
2. Organizational clarity: it makes defaults and thresholds discussable, reviewable, and maintainable as explicit policy references.

The set‑returning default is not a preference for large retained sets; it is a correctness safeguard when the order is not total. Single‑winner outcomes remain possible, but only by explicit criteria or declared admissible comparators.