On a diagram, Structured Activity Nodes have broken borders, as shown.
You can nest other elements underneath the Structured Node, including other Structured Activity elements such as Conditional, Loop and other Structured Node elements.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.476) states:
A StructuredActivityNode is an Action that is also an ActivityGroup (...) and whose behavior is specified by the ActivityNodes and ActivityEdges it so contains. Unlike other kinds of ActivityGroup, a StructuredActivityNode owns the ActivityNodes and ActivityEdges it contains, and so a node or edge can only be directly contained in one StructuredActivityNode. StructuredActivityNodes may be nested (as a StructuredActivityNode, as an Action, is also an ActivityNode), however, so an edge or node may be indirectly contained in a number of nested StructuredActivityNodes.