5 is hot, 6 is medium (still pretty hot) 7 is cold and 8 is really cold.

Plus you have long and short bodied plugs. Long bodied plugs are a bit hotter, so a long bodied 7 will be hotter than a short bodied 7 using the same wire for both. Not sure what Picco's letter designation is for those but that is what the different letters mean.