A command is a special marker used to access TopLeaf functions that are not available through the standard mapping dialogs. A command within custom content looks and behaves exactly like a tag in an XML document. Commands have start, end, and empty forms just like tags, and they (together with custom markers) form an XML-like hierarchy so that the content forms an XML fragment. Details of this structure are given in “Custom content structure”.

There are three classes of commands, Typesetting, Flow Control, and System. These are discussed in sections “Spaces, rules and fills”, “Flow control” and “The typesetting environment”.