These are the official zones of Carmel Valley North Maintenance Assessment District (MAD) as used by the Planning Board. MAD was formerly known as LMD (Landscape Maintenance District), so you might hear both terms used. These zones are used in determining assessments made on residents for maintaining certain community improvements. The improvements consist of parks, landscaped street medians and other community open space. Note also that this is only for north of SR-56, the southern part of Carmel Valley has its own MAD.
The numbering doesn't really make a lot of sense, it's really just something you have to memorize.