Check out the extract from Open XML Specification:
17.13 Annotations
Within a WordprocessingML document, annotations refer to various types of supplementary markup which can be stored inside or around a region of text within the document's contents.
[Example: The types of supplementary information stored within a
document can include: comments, revisions, spelling and/or
grammatical errors, bookmark information and optional editing
permissions. end example]
Within a document's contents, annotations
are stored in one of three different methods:
- Inline
- Cross Structure
- Properties
These three forms are needed in order to
maintain compatibility with both the legacy annotations functionality
of current word processing applications and the requirements of an
XML-based format (i.e. wellformedness of the resulting XML markup).
These three forms are referenced within the individual annotation
types described in the following sub clauses.
The whole section of 17.13
deals with Annotations
and can be read from - http://www.ecma-international.org/publications/standards/Ecma-376.htm