I had a hard time understanding what part of an image the grades (00 - 5) affected on a particular image I was working on. I was trying split grade printing. It wasn't until I made a test strip of the same area of a negative using each of the whole numbered filters (0,1,2,3,4,5) that it was clear what was happening. Try it you will be amazed at the results! It will also be very obvious what each grade does. Note, as others have pointed out, it is mostly dependent on the negative you are working on.
Like they say 'Seeing is believing.'