The paper describes Xdefinition 2.1 as an integral instrument for the design and development of projects with XML objects. In our development of Xdefinitions we concentrated on their application in the process of XML documents throughout the life cycle of the information system creation. This cycle typically includes discussions and negotiations with partners, analysts and implementators. We emphasized comprehensibility for all participants of the individual stages of IS formation as well as the binding character. The description of XML objects structures by means of Xdefinitions is easy to understand not only for informatics experts but also for a wider spectrum of participants who inevitably take part in the project implementation. By the binding character we mean the usability of the description for machine processing (description of data structures includes also processing code). Compared to the previous versions the Xdefinitions 2.1 thus allow also to define links of the objects to different situations in the course of their processing.