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Library / First Principles Framework (FPF) - Core Conceptual Specification
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C.23:4.4 - Unknowns & Shift classes (tri‑state discipline)

U1. (LEX). Enumerations for Degrade(mode) and Maturity rungs MUST be declared as closed value sets and registered at UTS (twin labels). Lexical SD (E.10) applies. U2. A live S2 characteristic or predicate admits unknown only when its C.22 value rule permits it; unknown MUST map to a branch (Degrade or Abstain) declared on the family (no coercions). Each branch publishes a branch‑id and (where used) a mode from a closed enum registered at UTS (LEX enum clarity). U3. ShiftClass semantics follow C.22. If ShiftClass ∈ {covariate‑shift, concept‑drift, adversarial} or unknown, default outcome is Degrade(scope‑narrow) unless a CAL.AcceptanceClause explicitly guards the regime.