On a diagram, Structured Activity Nodes have broken borders and composite diagram icons, as shown below:
To display the Activity diagram represented by a Structured Activity Node element, double-click on the element.
Structured Activity Node elements can point to child diagrams that themselves contain or consist of Structured Activity elements; that is, the Structured Activity elements are nested, as shown in the section of Project Browser below.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 409) states:
A structured activity node is an executable activity node that may have an expansion into subordinate nodes as an ActivityGroup. The subordinate nodes must belong to only one structured activity node, although they may be nested.
A structured activity node represents a structured portion of the activity that is not shared with any other structured node, except for nesting.