The three access log field names below are related to the proxy Application Classification feature.
x-bluecoat-application-name
x-bluecoat-application-operation
x-bluecoat-application-groups
If the feature is disabled, the access logs will show "Unavailable" as the value.
Go to Administration > Data and Cloud Services > Application Classification > Enable it.
By right, after enabling it, the proxy will start to download the database and you can see the status below.
Once the download completes, you could test the URL like what I did below, and from the access log, those "Unavailable" values should show as follows for login.windows.net.
x-bluecoat-application-name - Office 265 General
x-bluecoat-application-operation - none
x-bluecoat-application-groups - Online Productivity Suite