Distributed System Designers and DSI
The Dynamic Systems Initiative (DSI) is an effort simplify the design, development, and deployment of distributed applications. It's focused on system administration and operations. DSI consists of tools for developers and IT folks, and an underlying model (called the System Definition Model) to create definitions of distributed applications -- how they're partitioned, how they communicate, and constraints on those elements.
Well, the Distributed System Designers are an abstraction over this underlying SDM model. The different types of diagrams (Application, System, Logical Datacenter diagrams) all model different layers of the SDM. Many of the properties that we expose on the diagram map directly to information in that model. As a matter of fact, if you've used any of the designers, you may have noticed that there are .sdm files created in each project in your solution. Those files are the description of each piece of your distributed application.
Using the SDM descriptions of your application and your datacenter, we are able to validate that a given application meets the constraints of the machines on which it will be deployed. Plus, as this initiative grows, more tools will be written (by Microsoft and third-party vendors) to consume SDM and perform all sorts of operations and checks on it. The SDM files that the Distributed System Designers produce will be consumeable by those tools.
Therefore, if you're looking at creating applications that meet the DSI standards, using the Distributed System Diagrams is a good place to start.