Diagram state. Stuff common to all entities; an identifier, a name, an optional schema reference, and some optional notes. Optional annotations; non-structural information attached to any entity. Entity identifier. Optional reference to schema definition, where this entity corresponds to (and is constrained by) a schema entity. Human-readable name. ID of entity in originating system. For external use; not used by the sequencer widget. Diagram metadata, including name, identifier and schema reference. Definition of a single lifeline. A set of lifelines. May be top-level or in a fragment. An occurrence at one or other end of a message. A fragment directive. Whether fragment starts; fragment activated when @start=true. Indication of the last message in this fragment. Fragment operation. Start with the three everybody knows, but there are others. Guard condition. A message between lifelines. Guard condition within a fragment. Some fragments have more than one section, each with their own guard condition. An ordered set of messages and subsequences.