Meaning of the first digit : General growing place :
1 : WOODLAND. Plants, that in a garden need to be planted close to trees or shrubs. They require shade and prefer a peaty soil.
Meaning of the second digit: Specific demands :
2 : Plants for a sheltered, shaded place, between or near trees or shrubs, with a cool soil and maybe an hour or so sun a day.
Meaning of the third digit : Where to put :
2 : Not too sunny, for instance in a darker corner of a border.