The software architecture is the key core asset in any product line. The architecture defines the structures, relationships, and attributes for the product. A product line software architecture defines Variation Points that support tailoring the architecture definition to fit a specific product. 

More information on software architecture can be found at: http://www.sei.cmu.edu/architecture/index.html.

Each interface specification is used as the source of guidance for the developer assigned to develop any module that implements the interface. Testers also use the interface specifications as a source when generating black-box type test cases.