338 CHAPTER 13 Modeling Text-Based Requirements arrowhead pointing from the element that represents the more precise representation to the element being refined. An alternative compartment notation is also shown to represent this relationship. Note that the Weather Model state machine only partially refines the requirement. The Detection Scenario use case might address, for example, specific detection expectations in each weather condition. 13.14 USING THE GENERAL-PURPOSE TRACE RELATIONSHIP A trace relationship provides a general-purpose relationship between a requirement and any other model element. This is also discussed in Chapter 6, Section 6.8. The trace semantics do not include any constraints and therefore are quite weak. However, the trace relationship can be useful for relating requirements to source documentation or for establishing a relationship between specifications in a specification tree (refer to Chapter 17, Section 17.3.7). As shown in Figure 13.20, the trace relationship is used to relate a particular requirement to a Market Survey that was conducted as part of the needs analyses. The trace relationship is shown with a dashed line with the keyword «trace» with the arrowhead pointing to the source document. The survey is represented as a user-defined model element with the keyword «document» .