Examples: 1B  2-1-3; 1A; 2  1-2-1...

The first code number (such as "1B") refers to the plant's status.  The 1A code means the plant is extinct or extinct in the wild, where as a 1B means threatened, a 2 means vulnerable, 3- some concern, need more data and 4- limited distribution, not endangered (such as "only found on  the Channel Islands").

The second set of numbers (such as "2-1-3")  is the RED classification. The R-E-D classification stands for RARITY - ENDANGERMENT - DIVERSITY.  The higher the number, the more endangered the plant is, thus a 3-3-3 is generally a Federally endangered plant, where as 2-2-1 number would be a  "Endangered", "Threatened" or simply "Rare" plant as classified by the CNPS (California Native Plant Society).  Having a classification code means the plant is in some survival trouble nonetheless.

For a more detailed view of the RED List, try this link