Meaning of the first digit : General growing place :
3 : OPEN SPACES. Plants for an open space, free of trees and shrubs. Always in the sun and a more or less rich soil.
Meaning of the second digit: Specific demands :
2 : Plants for a warm, sunny place, that have the effect of borderplants (with which they combine very well).
Meaning of the third digit : Where to put :
7 : Plants that need protection against too much water in winter. Partly used as rock-garden plant.