On a diagram, Sequential Activity Nodes have broken borders and composite diagram icons, as shown below:
To display the Activity diagram represented by a Sequential Activity Node element, double-click on the element.
OMG UML Specification:
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 408) states:
A sequence node is a structured activity node that executes its actions in order.