When separate implementations independently derive identifiers and later compare them for equality, interoperability depends on more than the final identifier syntax or digest algorithm. The implementations need common semantics for which values are in scope, which distinctions matter to the comparison, and how identifiers are derived. Otherwise values intended to compare as equivalent can produce different identifiers, or processing can erase a relevant distinction before a later lossy or cryptographic operation is reached. This document gives principles for specifications that define such identifiers. It covers the comparison domain and equivalence relation, complete derivation semantics, the semantics under which output equality is interpreted, and processing that can create false matches before properties of a later operation are relied upon. It complements identifier comparison and hash-based naming specifications by focusing on the specification boundary from source values to independently reproducible identifiers. The document does not define a new identifier syntax, canonicalization format, resolver, registry, trust framework, or hash procedure.