+1 what Yooper said. Hulls are similar to the husk on barley. Typically you'd use them when using a significant amount of huskless grain (e.g. malted wheat, malted rye, etc) in the mash. They're mixed in with the grain and allow liquid to flow through the grain bed. If you're not using a significant amount of huskless grain, you can probably skip the hulls...but flaked rice would definitely not have the same effect as hulls.