What Pathology grading scale is used?

Hello, 

Thank you for making this extensive dataset publicly available for research!

I had a question regarding the pathology grading. As far as I’m aware, malignancy grading is typically performed on a 1 to 4 scale, but I noticed that your dataset uses a scale ranging from 1 to 6. Are grades 5 and 6 extensions of the standard system or part of a different type of pathology grading?

Thanks!