A Sequence Flow is used to show the order that activities will be performed in a Process. Each Flow has only one source and only one target. The source and target must be from the set of the following Flow Objects: Events (Start, Intermediate, and End), Activities (Task and Sub-Process), and Gateways. During performance (or simulation) of the process, a Token will leave the source Flow Object, traverse down the Sequence Flow, and enter the target Flow Object.
BPMN does not use the term “Control Flow” when referring the lines represented by Sequence Flow or Message Flow. The start of an activity is “controlled” not only by Sequence Flow (the order of activities), but also by Message Flow (a message arriving), as well as other process factors, such as scheduled resources. Artifacts can be Associated with activities to show some of these other factors. Thus, we are using a more specific term, “Sequence Flow,” since these lines mainly illustrate the sequence that activities will be performed.
A Sequence Flow MAY have a conditional expression attribute, depending on its source object. This means that the condition expression must be evaluated before a Token can be generated and then leave the source object to traverse the Flow. The conditions are usually associated with Decision Gateways, but can also be used with activities.
If the source of the Sequence Flow is an activity, rather than Gateway, then a Conditional Marker, shaped as a “minidiamond”, MUST be used at the beginning of the Sequence Flow (see Figure)
The diamond shape is used to relate the behavior to a Gateway (also a diamond) that controls the flow within a Process.
A Sequence Flow that has an Exclusive Data-Based Gateway or an activity as its source can also be defined with a condition expression of Default. Such Sequence Flow will have a marker to show that it is a Default flow.