Constraint expresses rules that apply universally, regardless of who invokes them. Compliance requirements, physical laws, organizational policies, and regulatory mandates are all Constraints. Unlike Authority, which differentiates between actors, Constraint binds all actors equally.
Constraints are implemented as SHACL shapes in a dedicated shapes graph, separate from the class ontology. The shapes graph operates under the Closed World Assumption: what is not explicitly permitted is prohibited. At runtime, the shapes graph validates primitive instances against constraint definitions, and violations produce Evidence records (validation reports with truth type Authoritative). This separation enables profile layering — different substrate profiles (§21) apply different constraint sets to the same primitive classes without modifying class definitions. Each constraint specifies an enforcement mode: Blocking (prevents the operation), Warning (allows but alerts), Logging (records silently), or Advisory (suggests alternatives).
Semantic boundary: Constraint is the rule; Authority is the right. Constraint answers "is this permitted regardless of who is asking?" — Authority answers "who is allowed to decide?" A constraint applies to all actors uniformly; an authority delegation applies to specific actors within specific scopes.