Relating requirements with trace links

A trace link creates a directional relationship between two objects, in this case, requirements. The relationship is conveyed by the terms defining and complying. A defining object specifies a condition that a product or a component must fulfill. A complying object partially or completely fulfills a condition specified by a defining object. Such a relationship establishes a path in which one object precedes the other.

For example, functional requirement A, for target tracking, defines hardware requirement B, for a CPU with a certain instruction rate. In this example:

• Requirement A precedes requirement B, with requirement B directly downstream in the complying path.
• Requirement B succeeds requirement A, with requirement A directly upstream in the defining path.

(Diagram: defining/complying trace link relationship — not reproduced)

A requirement can assume both defining and complying relationships, continuing the path upstream and downstream. For example, a weight requirement may define a requirement to use aluminum. That complying requirement may in turn define temperature or environmental requirements consistent with the properties of aluminum. A requirement can have any number of defining and complying trace links.

Trace links can be created between requirements within a single project and also between requirements in two different projects. This two-way relationship allows for change analysis in both directions.

If you are considering a change to a product requirement, you can follow its trace links downstream to all complying requirements that were derived from the product requirement and eventually to all design elements that must comply. Conversely, an engineer working on a design element that must meet detailed, low-level requirements can follow those trace links upstream to the original customer requirements that define the design constraints.