"Content Model Interoperability is centered around enabling all or part of one standard's content model to be included as part of another standard. For example, DITA's specialization capabilities could be employed to create custom topic types for DocBook sections or refentries (in a DITA-like way). Conversely, DocBook's DTDs are designed to create customizations on top of the core content model."

from http://jims-thoughtspot.blogspot.com/2007/02/types-of-xml-content-interoperability.html