The numbers on the product names are related to the thermal conductivity of the product. So the lower the number, the better thermal performance that product will give. For example, in the Knauf DriTherm range, the "Therm 37" relates to the product's thermal conductivity value of 0.037 W/mK whilst the 'Dri' refers to the slab's water repellent additive, added to maximise its overall thermal performance.