At
http://www.agilemodeling.com/style/classDiagram.htm#_Toc855100 and below I post guidelines regarding the various relationships (assocation, aggregation, and composition).
Frankly, my advice is to not worry all that much about what type of relationship it actually is. For example, one poster said that with composition you always delete the part but not with association or aggregation. Many people would disagree with that, and say that you would automatically do that as well for aggregation too. The point is that different people have different styles of working, so if you choose to impart meaning into what the diamonds represent then everyone on the team had better agree to a common set of modeling style guidelines.
- Scott
<a href="http://www-306.ibm.com/software/rational/bios/ambler.html" target="_blank" rel="nofollow">Scott W. Ambler</a><br />Practice Leader Agile Development, IBM Rational<br /> <br />Now available: <a href="http://www.ambysoft.com/books/refactoringDatabases.html" target="_blank" rel="nofollow">Refactoring Databases: Evolutionary Database Design</a>