Skip to main content


Invisible informations about content elements

Tags are invisible labels which mark content elements as a special structure type. Experts call this semantic markup. This means: the labels indicate the role of content elements in the context of the document. Typical roles are for example:

  • heading level 1,
  • paragraph,
  • list,
  • table,
  • figure or
  • caption

Machine-readable content

Tags make content machine-readable. Assistive technologies need machine-readable content. It is an important precondition for accessible content.


You can put such invisible informations into a PDF. A PDF-Tag consists of the structure type and the related marked content element. To keep it simple the term "tag" is often used equivalent to the term "structure type". All tags in a document form the tag tree.

Similar terms: structure information, label, structure type, semantic markup