A structured diagram is a set of logical relationships between items, where these relationships provide visual organization or describe special interaction behaviors between them.
The Microsoft Visio team analyzed the requirements for adding structure to diagrams, and came up with a number of features that needed to be added to the Visio product to achieve this:
Container Management: The ability to add labeled boxes around shapes to visually organize them
Callout Management: The ability to associate callouts with shapes to display notes
List Management: To provide order to shapes within a container
Validation API: The ability to test the business logic of a diagram
Connectivity API: The ability to create, remove, or traverse connections easily
The following diagram demonstrates the use of Containers and Callouts in the construction of a basic flowchart, which has been validated using the Validation API, that in turn uses the Connectivity API.