Part 1: Fundamentals and Markup Language Reference Describes concepts for documents and applications of both strict and transitional conformance. Part 2: Open Packaging Conventions Defines requirements for documents, producers, and consumers. Part 3: Markup Compatibility and Extensibility Describes a set of conventions for forward compatibility of markup specifications, applicable not only to Office Open XML specifications as described in Parts 1 and 4 of this Standard, but also to other markup specifications. Part 4: Transitional Migration Features Specifies features for backward-compatibility and that are useful for high-quality migration of existing binary documents to ECMA-376.