XML documents don't live in a vacuum . They are parts of larger systems. They are written in and read by a variety of software tools, transmitted across networks in a variety of formats using various protocols, and stored in file systems and databases. They interact with other processes. A single document may be read and written by many different people and programs during its existence, each of whom may interpret it in a different way.
This final part explores issues that arise when looking at XML documents as parts of larger systems. These include verification, data integrity, compression, authentication, caching, content management, and the like. Sometimes these services can be performed in the external processes that manage, manipulate, and transmit the XML documents. Other times the need for these services requires changes in the design and structure of the XML documents and applications themselves .