Most people using Report Builder 2.0 with a report model soon ask the question, what is that extra column I get back with strange values like “AC8AAAA=” in it?

Answer: It's an entity key value. If you’re going to create a group on that entity in your report, you should use this field as the group expression. This is especially important if you’re going to create Clickthrough links, because those key values will be used to filter the target report, and filters on keys usually have better performance.