There are many different XML schema languages, and in the publishing industry, complex content models like DocBook and TEI are represented through schemas, each addressing specific content requirements:
DocBook: now defined using the RELAX NG schema language, offers cleaner, more precise, and more extensible content models, making customised versions significantly easier to create than its previous DTD-based schema. [D5TDG]
TEI: The Text Encoding Initiative is an XML application designed for the markup of classic literature, widely used by libraries, museums, publishers, and individual scholars to represent all textual material for online research and teaching. The guidelines define and document a markup language for representing the structural, renditional, and conceptual features of texts. [TEI]
VRA (Visual Resources Association) Core: An XML-compatible metadata schema for organising works of art, namely visual resources.
For specific publishing workflows, we implement a customisation layer to our structural schemas by extending core schemas such as the DocBook Publishers schema [DBR]. This customisation is achieved with the use of an include statement in our customised schemas, and it allows us to build on the core standard structures whilst incorporating specific requirements and constraints that align with our publishing requirements [CDB]. Over time, the customised schemas have evolved to address the increasing complexity of the content we process; therefore, our commitment to ensuring comprehensive and accurate validation throughout the content lifecycle is vital.