It is seconds per mile
A 180 boat is 20 seconds per mile faster than a 200 rated boat and 20 seconds per mile slower than a 160 rated boat. So if you do 10 mile race, the 180 boat needs to finish 200 seconds ahead of the 200 boat and within 200 seconds afte the 160 boat to beat both of them on "corrected" time. I forget all the variables that go into the rating, but they include measurments of the sails actually on the boat. For example the headboard on the main is measured and the largest headsail is measured. Maybe Google PHRF to see if your model boat has been rated. This would give you a range of ratings depending on the "as sailed" details of each boat.