Content items are graphical objects that exist in the document independently of the structure tree but are associated with structure elements.
Content items are leaf nodes of the structure tree.
SElement represents PDF structural elements, which are nodes in a tree
structure, defining a PDF document's logical structure.
Unlike the StructTree, SElement can have two different kinds of children: another SElement or a ContentItem (which can be marked content (MC), or a PDF object reference (OBJR)).
STree is the root of the structure tree, which is a central repository for information related to a PDF document's logical structure. There is at most one structure tree in each document.
Content Item types