This Part of ECMA-376 specifies a set of conventions that are used by Office Open XML documents to define the structure and functionality of a package in terms of a package model and a physical model. The package model is a package abstraction that holds a collection of parts. The parts are composed@ processed@ and persisted according to a set of rules. Parts can have relationships to other parts or external resources@ and the package as a whole can have relationships to parts it contains or to external resources. The package model specifies how the parts of a package are named and related. Parts have content types and are uniquely identified using the well-defined naming rules provided in this Part of ECMA-376. The physical mapping defines the mapping of the components of the package model to the features of a specific physical format@ namely a ZIP archive. This Part of ECMA-376 also describes certain features that might be supported in a package@ including core properties for package metadata@ a thumbnail for graphical representation of a package@ and digital signatures of package contents. Because this Part of ECMA-376 might evolve@ packages are designed to accommodate extensions and to support compatibility goals in a limited way. The versioning and extensibility mechanisms described in Part 3 support compatibility between software systems based on different versions of this Part of ECMA-376 while allowing package creators to make use of new or proprietary features. This Part of ECMA-376 specifies requirements for documents@ producers@ and consumers. Conformance requirements are identified throughout the text of this Part of ECMA-376. A formal conformance statement is given in ?? An informative summary of requirements relevant to particular classes of developers is given in Annex H.