Subject classification with DITA and SKOS

DITA (Darwin Information Typing Architecture) has been developed by IBM since 2001, and introduces itself as a "topic-oriented architecture". DITA has its own definition of a topic, which is a bit different, and in a sense more restrictive, than the one(s) found in Topic Maps.
A topic is a unit of information that describes a single task, concept, or reference item.
The new publication, really worth reading, comes with a challenging academic subtitle : "Managing formal subjects", hiding in fact a very pragmatic approach:
In a topic-oriented architecture such as DITA, content is authored in small, independent units that are assembled to provide help systems, books, courses, and other deliverables. Each unit of information answers a single question for a specific purpose. That is, each topic has specific, independent subject matter -- the very reason that these units of information are called topics.
The paper then expands very neatly on how SKOS can be used to declare what the subject of a topic is, claiming that "subject" here is to be understood in the same sense than in "Published Subject Indicator".