All tags together in a document form a tree with a hierarchy and a clear logical order. In this regard a tag can have parents (superior elements) or childs (inferior elements).
In a PDF document the tag tree is invisible, but you can see it in a special view like
- the view Structure Tree in axesPDF or
- the view Logical Structure in PDF Accessibility Checker (PAC).
Similar terms: structure tree, structure view, logical structure, logical document structure