You can reuse content not just for print and the web but also for multiple custom outputs. Reuse at a heading or line level can be chaotic and challenging to implement and manage, especially at an enterprise-wide level. DITA facilitates reusability at a more manageable content level: the topic. A topic is the smallest chunk of information that can stand alone as meaningful information. Topics are then assembled into documents using Ditamaps, which are hierarchical lists of pointers or links to topics. These pointers are called <topicref> elements.
Topics can also be reused in other topics. Each topic is assigned a unique ID. From a topic, you can include a content reference (conref) to another topic using its unique ID. At a finer level of granularity, you can also assign property tags to individual topic elements for conditional assembly.