Here's a look at the King County zoning codes and what they mean:

Development Conditions:
Base zoning codes may be modified by one of more of the following development codes:

P: Parcel-specific condition
SO: Special district Overlay
DPA: Demonstration Project Area

Base Codes:
"DU" stands for "Dwelling Unit"

A-10
Agricultural, one DU per 10 acres
A-35
Agricultural, one DU per 35 acres
F Forest
M Mineral
RA-2.5 Rural Area, one DU per 5 acres (not a typo)
RA-5 Rural Area, one DU per 5 acres
RA-10 Rural Area, one DU per 10 acres
UR Urban Reserve, one DU per 5 acres
R-1 Residential, one DU per acre
R-4 Residential, four DU per acre
R-6 Residential, six DU per acre
R-8 Residential, eight DU per acre
R-12 Residential, 12 DU per acre
R-18 Residential, 18 DU per acre
R-24 Residential, 24 DU per acre
R-48 Residential, 48 DU per acre
NB Neighborhood Business
CB Community Business
RB Regional Business
O Office
I Industrial