? Object: The interaction among objects takes place in a system. An object is depicted by a rectangle with the name of the object, preceded by a colon and underline.
? Relation/Association: Association between objects is linked by connecting them. The cardinality can be depicted by placing qualifiers on either ends.
? Messages: An arrow that beginning from one object to the destination object. This depicts the interaction among objects. The sequence or order of the interaction is depicted by the number.