Links represent relationships that exist between symbols.

A Message Flow shows the flow of messages between two model elements that are prepared to send and receive them. A model element in this case is either an organizational unit, a function, an application, a class, an entity, or a location. The model elements is represented on the BPMN Business Process diagram by a pool or swimlane.

Messages are sent between two different departments or organizations. System Architect enforces this rule by mandating that a Message Flow line be drawn across a pool or a lane for it to connect between two processes. If you try to connect a Message Flow line between two processes in the same lane, System Architect does not allow the connection.

A Message Flow line can be drawn to a process in a pool or lane, or to the edge of the pool or lane itself.

You cannot draw a Message Flow to or from a Gateway.

A Sequence Flow is used to show the order that activities will be performed in a process flow. Sequence Flows are also used in conjunction with the drawing of a Fan-In or Fan-Out Gateways symbols. In the definition dialog there is a Sim Symbol tab that will allow you to associate what type of gateway logic that will be used. There are several types available: Mandatory, Event Based, Attribute, Object Type, Probability, Shortest Queue, and Time in Model. When the gateway type has been established, select the Apply button to reveal the appropriate gateway tab.

You can draw a Sequence Flow between two processes in a pool. You cannot draw a Sequence Flow between a process in one lane and a process in another lane.