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