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 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.