| 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. |