Which element is not the most important element in DITA? Paradoxically, that would be the <dita> element. This element defines a container that can hold one or more DITA topics, but it has no formal meaning as a part of structured representation. You might use it to group several topics in a single file unit for ease of loading and working on in parallel, particularly when using file-based content in a word processor-based DITA editor such as FrameMaker or Simply XML or Quark XML Author.
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You can address topics in a <dita> container individually from a topicref, meaning that you can actually reference them in any order or preferred nesting for your actual output. You can also reference them by conref to replace other topics, as needed. In this sense, the <dita> container could be used as a sort of bin or warehouse for topics-by-category for more effective file-based storage and organization. At some point, CMS-based categories become more useful, at which point the usefulness of the <dita> element for grouping becomes more of an artificial hindrance. Use it appropriately, but as with all things good, use it in moderation!