Alfresco is capable of handling multiple classifications, or hierarchies of classification, it's a very useful feature, and can make your life a lot easier when looking for documents, especially the ones with no indexed content like pictures, scanned documents etc...
Classifying a document in Alfresco can be as easy as few clicks on the browser, however it can be very time-consuming process if you are uploading many documents every day, or if you are migrating to Alfresco : Imagine having to manually classify a few thousands of documents!
If you are still classifying documents manually, analyzing their content, and sorting them into categories, you might be interested in finding out how you can extend Alfresco to automatically classify your documents for you.