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 :
3 : Plants for a sunny, but somewhat cool place.
Meaning of the third digit : Where to put :
6 : Plants to be used in combination with rock-garden plants.