Entries in this collection are not interpreted by the RichTextDocument. Typical usage of this property is to store custom attributes for your own purposes. Depending upon the underlying document format, these attributes will be saved and restored with successive load of the document.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Server 2012, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2