2.24.5 Non-Simple Property Set Storage
A non-simple property set MUST be represented by an alternate stream with a name derived from the standard name specified in section 2.23 according to the following ABNF [RFC4234] syntax.
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NonSimplePropertySetAlternateStreamName = "Docf_" PropertySetStreamOrStorageName
This stream's contents MUST be in the format of a compound file (see [MS-CFB] for details). The root storage of this compound file MUST conform to the non-simple property set storage format as specified in section 2.22.