A Sequence Flow is used to show the order of flow elements in a process or a choreography.
The source and target of a Sequence Flow must be from a set of the following elements:
Events (Start, Intermediate, and End events)
Activities (Task and SubProcess for Processes)
Choreography Activities (Choreography Task and Sub-Choreography)
A Sequence Flow can optionally define a condition expression indicating that a token will be passed down the Sequence Flow only if the expression is evaluated to be true.
A Condition expression is typically used when the source of a Sequence Flow is a Gateway or an Activity. A conditional outgoing Sequence Flow from an Activity is with a mini-diamond (indicator) at the beginning of the Sequence Flow.
A Conditional Sequence Flow outgoing from a Gateway does not have a mini-diamond at the beginning of the Sequence Flow.
A Sequence Flow, which has an exclusive, inclusive, or complex gateway, or an Activity as its source, can also be defined as a default Sequence Flow. The default Sequence Flow is represented with a backslash.
A default Sequence Flow will be taken (a token is passed) only if all of the other outgoing Sequence Flows from an Activity or Gateway are not valid, meaning that their condition expressions are false.