When do we use the app access token and user access token?

Take for example, when using the Koala ruby gem

To access the graph api, we initialize the graph object using the user token key. This is clear, since it contains information that we got from the successful authentication

To access the realtimeupdates api, we initialize the realtimeupdates object using the app access key, which does not seem to have require any user specific authent开发者_Python百科ication information

So, when does facebook require which token?

Here is a detailed explanation on OAuth site. Link here