When is it appropriate to use CacheItemRemoveCallBack?

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ASP.NET

cache

Hi friends,
As per my knowledge CacheItemRemoveCallBack is a delegate and provides an indication to the application about the cached item has been removed.

I have few question

1. when it is fired?
2. whats is diff between CacheItemRemoveCallBack and CacheItemUpdateCallback? 
3. where is write place to get the fresh data from the DB, CacheItemRemoveCallBack/CacheItemUpdateCallback?

Thanks in advance