This access log field is a new field that's been added starting in 6.2 version, and provides the application name. For example when a user is using Facebook and is posting a message:
So the 'application' is the x-bluecoat-application-name field.
However you need to use BCWF as your content filter database. Other content filter databases such as websense cannot be used, and will record the value as 'Unavailable' or '-'.