This part of ISO/IEC 9126 describes a two-part model for software product quality: a) internal quality and external quality, and b) quality in use. The first part of the model specifies six characteristics for internal and external quality, which are further subdivided into subcharacteristics. These subcharacteristics are manifested externally when the software is used as a part of a computer system, and are a result of internal software attributes. This part of ISO/IEC 9126 does not elaborate the model for internal and external quality below the level of subcharacteristics. The second part of the model specifies four quality in use characteristics, but does not elaborate the model for quality in use below the level of characteristics. Quality in use is the combined effect for the user of the six software product quality characteristics. The characteristics defined are applicable to every kind of software, including computer programs and data contained in firmware. The characteristics and subcharacteristics provide consistent terminology for software product quality. They also provide a framework for specifying quality requirements for software, and making trade-offs between software product capabilities. Normative Annex A provides recommendations and requirements for software product metrics and quality in use metrics. Examples of these metrics are contained in other parts of ISO/IEC 9126. These metrics are applicable when specifying the quality requirements and the design goals for software products, including intermediate products. An explanation of how this quality model can be applied in software product evaluation is contained in ISO/IEC 14598-1. This part of ISO/IEC 9126 enables software product quality to be specified and evaluated from different perspectives by those associated with acquisition, requirements, development, use, evaluation, support, maintenance, quality assurance and audit of software. It can for example be used by developers, acquirers, quality assurance staff and independent evaluators, particularly those responsible for specifying and evaluating software product quality. Examples of uses of the quality model defined in this part of ISO/IEC 9126 are to: • validate the completeness of a requirements definition; • identify software requirements; • identify software design objectives; • identify software testing objectives; • identify quality assurance criteria; • identify acceptance criteria for a completed software product. NOTE 1 This part of ISO/IEC 9126 can be used in conjunction with ISO/IEC 15504 (which is concerned with the software process assessment) to provide: • a framework for software product quality definition in the customer-supplier process; • support for review, verification and validation, and a framework for quantitative quality evaluation, in the support process; • support for setting organisational quality goals in the management process. NOTE 2 This part of ISO/IEC 9126 can be used in conjunction with ISO/IEC 12207 (which is concerned with the software lifecycle) to provide: • a framework for software product quality requirements definition in the primary lifecycle process; • support for review, verification and validation in supporting lifecycle processes. NOTE 3 This part of ISO/IEC 9126 can be used in conjunction with ISO 9001 (which is concerned with quality assurance processes) to provide: • support for setting quality goals; • support for design review, verification and validation.