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The use of topics is up to you, until they become more useful. See Record Topics.
Advanced Setup / Dev
http://www.salesforce.com/us/developer/docs/case_feed_dev/
Page Layout
An example Case Feed Layout.
The Email screen
Using Case Feeds
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- Flick back to the Posts publisher action to free up some screen real estate and see more of the feed.
Emailing
The Email screen
http://help.salesforce.com/HTViewHelpDoc?id=case_interaction_sending_email.htm&language=en_US
Case Feed Articles
Note: Some of these articles may show features only related to Service Cloud, such as the Console.
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