The Trace relationship is a specialization of an Abstraction, connecting model elements or sets of elements that represent the same concept across models. Traces are often used to track requirements and model changes, typically in a Traceability diagram, or in a Class, Use Case, Object or Composite Structure diagram.
As changes can occur in both directions, the order of this Trace is usually ignored. The relationship's properties can specify the trace mapping, but the trace is usually bi-directional, informal and rarely computable.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.682) states:
Specifies a trace relationship between model elements or sets of model elements that represent the same concept in different models. Traces are mainly used for tracking requirements and changes across models. As model changes can occur in both directions, the directionality of the dependency can often be ignored. The mapping specifies the relationship between the two, but it is rarely computable and is usually informal.