Sequence Diagram – Node Types

In a Sequence diagram, there are five Node Type options: Actor, Lifeline, Lifeline Control, Lifeline Boundary, and Lifeline Entity. Examples of each are provided below.


The Actor node type is the default selection and typically characterizes the role that initiates the interaction being modeled. The Actor type is also used when the Sequence diagram is intended to illustrate part of a broader Use Case diagram.


The Lifeline node type is the most commonly used shape and represents a role in the modeled interaction. A role may be filled by a hardware system (such as a space vehicle) or a human user (such as a space operations center), but it does not denote a specific unit or individual like an Actor does.

Lifeline Control

The Lifeline Control node type represents the items that manage the flow of the modeled interaction. A control can communicate with boundaries (the Lifeline Boundary node type) and entities (the Lifeline Entity node type), but cannot communicate with actors. A control often corresponds to a broader Use Case diagram.

Lifeline Boundary

The Lifeline Boundary node type represents items that interface with actors, such as a computer screen or an application menu. A boundary can interact with controls and actors, but not entities.

Lifeline Entity

The Lifeline Entity node type depicts items that represent system data. This element is not "the data;" rather, it manages the behaviors around data. An entity can only communicate with a control.
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