||An application or plug-in extension that processes logical structure can attach
additional information, called attributes, to any structure element.
||The Class ClassMap.
||Content items are graphical objects that exist in the document independently
of the structure tree but are associated with structure elements.
||RoleMap is a dictionary that maps the names of structure types used in the
document to their approximate equivalents in the set of standard structure
||SElement represents PDF structural elements, which are nodes in a tree
structure, defining a PDF document's logical structure.
||STree is the root of the structure tree, which is a central repository
for information related to a PDF document's logical structure.