Office Open XML Native Extensibility Constructs

Clause 12 of Part 5 specifies the ability for a markup language to define additional constructs for extensibility of a specific markup language. Within the context of Office Open XML documents, the <extLst> element(s) defined in individual markup languages shall allow the round-tripping of all unknown content regardless of the state of the <PreserveElements> and <PreserveAttributes> elements. See Part 4 for additional information on the valid use of the <extLst> construct.