Create specialized topics when you have a restrictive category of topics that you want to keep consistent and that your users want to distinguish from other categories. Create specialized domains when you have a set of elements that you want available across several of your topic types. Be sure to specialize from the correct base: For example, categories of reference topics should specialize <reference>, categories of tasks should specialize <task>, and domain types should always specialize either <topic> or another domain type. If you need to allow more content structures than the base types allow, you can specialize directly from topic, or form your own base type. However, the lower down in the hierarchy that you can specialize, the better; you can then take advantage of any transforms or processes that have been developed for the more general categories that you specialize from.