3. Identifying DocBook Documents and Schemas reads:

Historically, when DocBook was defined by a DTD, DocBook documents could be identified by the presence of standard public and/or system identifiers in the document type declaration. RELAX NG, the normative schema language for DocBook V5.0, does not provide any equivalent mechanism.

For systems that can make use of public identifiers, e.g., systems where the informative DTD is being used, the following public identifier should be used for DocBook V5.1: “-//OASIS//DTD DocBook V5.1//EN//XML”.

The first paragraph is clearly non-normative and should be a note.

The second paragraph expresses a normative requirement and should appear as normative text.