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 Unified Modeling Language specification, (v2.5.1, p.480) states:
A SequenceNode defines a complete, sequential ordering of all the ActivityNodes it contains, which must all be ExecutableNodes. When the SequenceNode executes, each of the nodes within it are executed in sequential order. The SequenceNode may also contain ActivityEdges between its nodes, and ActivityEdges may cross into and out of the SequenceNode. The semantics are equivalent to a general StructuredActivityNode containing the same nodes and edges, but with ControlFlows added to sequentially order the nodes as specified for the SequenceNode.