A combined fragment defines an expression of interaction fragments. A combined fragment is defined by an interaction operator and corresponding interaction operands. Through the use of combined fragments the user will be able to describe a number of traces in a compact and concise manner.

A gate is a connection point for relating a message outside an interaction fragment with a message inside the interaction fragment.

An interaction use refers to an interaction. The interaction use is a shorthand for copying the contents of the referenced interaction where the interaction use is. To be accurate the copying must take into account substituting parameters with arguments and connect the formal gates with the actual ones.

A lifeline represents an individual participant in the interaction. While parts and structural features may have multiplicity greater than 1, lifelines represent only one interacting entity.

A message defines a particular communication between lifelines of an interaction.