That is to say, a document which has actually been authored conveys more meaning than just stating "I am a valid document relative to the specification".
in a closed-world environment as we have been discussing until now, that additional meaning has no explicit counterpart in the knowledge-base; it is only represented implicitly in the abstract content tree, in a form which is not perspicuous and would be difficult to re-use for the authoring of other documents or to share with other processes.