From http://msdn.microsoft.com/en-us/library/ff531265(v=office.12).aspx:

	Office reads and writes a set of well-defined Custom XML Parts. 

	Custom XML Parts, in general, are described in Custom XML Data Storage Part ("[ECMA-376] Part 1 §15.2.4; Custom XML Data Storage Part") and Custom XML Data Properties ("[ECMA-376] Part 4 §7.5; Custom XML Data Properties"). 

	In addition to implementing the relevant portions of ECMA-376 Office Open XML File Formats 
	[ECMA-376], Office understands a predefined set of schemas when applied to Custom XML Parts.
	
		Cover Page Properties
		coverPageProperties
		http://schemas.microsoft.com/‌office/‌2006/‌coverPageProps
		§3.5.1, Cover Page Properties
		
		Content Type Schema
		contentTypeSchema
		http://schemas.microsoft.com/‌office/‌2006/‌metadata/‌contentType
		§3.5.2, Content Type Schemas
		
		Custom Document Information Panel
		customPropertyEditors
		http://schemas.microsoft.com/‌office/‌2006/‌customDocumentInformationPanel
		§3.5.3, Custom Document Information Panel
		
		Custom XSN
		customXsn
		http://schemas.microsoft.com/‌office/‌2006/‌metadata/‌customXsn
		§3.5.4, Custom XSN
		
		Long Properties
		longProperties
		http://schemas.microsoft.com/‌office/‌2006/‌metadata/longProperties
		§3.5.5, Long Properties
		
		