The Trace relationship is a specialization of a Dependency, linking model elements or sets of elements that represent the same idea across models.  Traces are often used to track requirements and model changes, typically in a Class , Use Case, Object or Composite Structure diagram. As changes can occur in both directions, the order of this Dependency is usually ignored. The relationship's properties can specify the trace mapping, but the trace is usually bi-directional, informal and rarely computable.

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OMG UML Specification

The OMG UML specification (UML Superstructure Specification, v2.0, p. 17) states:

"A dependency that indicates a historical or process relationship between two elements that represent the same concept without specific rules for deriving one from the other."