I think it is the legnth in cm???? Not sure but it would work out approx. For example 1″ = 2.54 cm. So a #11 would be about 4.33″ long. A # 13 would be about 5.12″ long and so on.
The only ones I know for sure is the “DT series”, which means “Dives To”. So, if it is a DT6, the bait will run up to 6 ft deep in ideal conditions. DT16=16 feet, DT10=10 feet, and so on. Hope this helps, Chad.