Cedar siding grades explained? Two contractors quoted me 'the same thing' but the bids came back wildly different

Getting bids to re-side my house in western red cedar. One contractor is quoting a clear, knot free grade and another a knotty grade, and the gap between their bids is huge. Both just write 'western red cedar' on the quote. Obviously they aren't talking about the same product, but neither will clearly explain the difference. Can someone break down cedar grades for me so I know what I'm actually comparing?