A topic is a text snippet, and is the basic editable unit of an outline.
In Opal, a selection is always a topic (or topics) as a whole, or else text within a single topic.
A topic can be created or deleted. A topic’s text can be edited. A topic can be moved within the outline. A topic can be copied, cut, and pasted.
A topic’s text is styled text, and may include pictures and links to files on disk.