On a diagram, Sequential Activity Nodes have broken borders, and can contain nested elements that define a sequence of actions.
Sequential Nodes are flagged as composite elements in the context menu ('New Child Diagram | Composite'); however, when you add the child diagram the element converts to a simple composite Activity.
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.