1 = A: very firm cooking, no disintegration of the outer side of boiled tuber.
2 = AB: between 1 and 3.
3 = B: rather firm cooking but outer side of tuber starts to disintegrate.
4 = BC: between 3 and 5.
5 = C: boiled tubers start to disintegrate, tubers are falling apart.
6 = CD: between 5 and 7.
7 = D: tubers completely disintegrated. Looks like mashed potatoes.
Cooking types 5,6,7 are mainly varieties with very high dry matter like some crisping and the starch varieties. In table potatoes these types are not desired anymore.
Cooking type can vary depending from the area the potatoes are grown. Varieties with B cooking type in Holland can have AB or sometimes even A in the south of Europe. In general varieties are going more to the firmer types when they are grown more to the south of Europe.