A critical modeling challenge when analyzing and designing "information supply chains" is identifying the common elements in a set of related documents and organizing and encoding them at the optimal granularity for reuse
DTDs, with their roots in SGML for publishing, were well-suited for capturing structural hierarchy but not as effective in modeling component reuse
By 1998 Veo Systems, the original developers of xCBL, had realized that more object-oriented modeling capability than DTDs provided was needed to enable customization for specialized industry of company contexts in a controlled way that maximizes reuse
So Veo developed SOX, the Schema for Object-Oriented XML, which allowed for extensions of base schemas and other modeling innovations that were later incorporated into XSD, the W3C's schema language for XML
