Topics are the basis for high-quality information. They should be short enough to be easily readable, but long enough to make sense on their own.
By organizing content into topics, authors can achieve several goals simultaneously:
Topics are small enough to provide lots of opportunities for reuse, but large enough to be coherently authored and read. While DITA supports reuse below the topic level, this requires considerably more thought and review, since topics assembled out of smaller chunks often require editing to make them flow properly. By contrast, since topics are already organized around a single subject, authors can organize a set of topics logically and get an acceptable flow between them, since transitions from subject to subject don't need to be as seamless as the explanations within a single subject.
OASIS DITA Architectural Specification v1.0 -- 09 May 2005
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