追跡性 
参考定義
IEEE std 830-1998 (Traceable)
An SRS is traceable if the origin of each of its requirements is clear and if it facilitates the referencing of each requirement in future development or enhancement documentation. The following two types of traceability are recommended:
a)Backward traceability (i.e., to previous stages of development).
This depends upon each requirement explicitly referencing its source in earlier documents.
b)Forward traceability (i.e., to all documents spawned by the SRS).
This depends upon each requirementin the SRS having a unique name or reference number.
The forward traceability of the SRS is especially important when the software product enters the operation
and maintenance phase. As code and design documents are modified, it is essential to be able to ascertain the
complete set of requirements that may be affected by those modifications.
