Almost any content element in DITA can be made reusable simply by adding an id attribute to it. The DITA content reference (conref) mechanism can reuse any element with an id. However, it is considered best practice not to reuse arbitrary pieces of text out of arbitrary topics due to the management overhead this creates, and the possibility of authors deleting or changing content that is reused in other topics without being aware that the content is reused. Instead, create and manage a separate set of topics that contain topics designed specifically for reuse.
Oxygen XML Editor makes it easy to create reusable content components from existing topic content.
To ensure that the topic file that contains the reusable component is a valid container for just the reusable content component, without having to include the other elements required by a standard topic type, Oxygen XML Editor creates a specialized topic type on the fly. This specialization is designed to make sure that the content is compatible with the topic type from which it is created.
You now have a reusable component that you can include in other topics using a content reference or a content key reference.