Q: Is there a technical requirement for the single-file per topic attribute of DITA ?  (HTML can handle multiple topics per file, not sure why DITA doesn't.)

A: (Don Day) Be assured that DITA indeed allows multiple topics per file!  You might be confusing a best practice as a limitation of the architecture.

A topic is a chunk of discourse that supports a topical theme, which is usually expressed by the heading for that chunk.  HTML supports multiple levels of heading, so in this sense, it can contain multiple levels of topics.  However, in HTML, these "topics" are not bounded--the apparent relationship of the discourse to the heading is purely something that readers understand by convention, and the hierarchy is only visually implied through font sizes.  DITA also allows heading hierarchies through nesting--any topic type can nest additional topics of the same type. But in DITA, these topics are independently selectable units, unlike HTML.  And when using the ditabase DTD, you can actually nest topics of different types, as you might need to when writing an article or other more extensive kind of deliverable that is not, strictly speaking, an information unit for a help system.

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