This document is one of a series of guides to software engineering produced bythe Board for Software Standardisation and Control (BSSC), of the European SpaceAgency. The guides contain advisory material for software developers conforming toESA's Software Engineering Standards, ESA PSS-05-0. They have been compiled fromdiscussions with software engineers, research of the software engineering literature,and experience gained from the application of the Software Engineering Standards inprojects.