This site is from a past semester! The current version will be here when the new semester starts.
Can explain multi-level design 
In a smaller system, the design of the entire system can be shown in one place.
This class diagram of se-edu/addressbook-level2 depicts the design of the entire software.
The design of bigger systems needs to be done/shown at multiple levels.
This architecture diagram of se-edu/addressbook-level3 depicts the high-level design of the software.
Here are examples of lower level designs of some components of the same software: