Harris County State class/category code/ID (A1, B1, B2, etc)

[Nick S.]

In looking through the HCAD records for Harris County, I'm trying to figure out what to make of the code that seems to go by many names. In one place, it's called "State class code", in another place it's called "State category code" and in yet another it's called "State category ID". These are the codes such as "A1 - Real, Residential, Single-Family", or "B1 - Real, Residential, Multi-Family".

Are these codes just a reflection of how the property is being used? 

For example, if a property is categorized as "B2 - Real, Residential, Two-Family" and you wanted to add a third unit on the property, what's the process? I know you would have to operate within any deed restrictions but other than that, how would you change to "B3 - Real, Residential, Three-Family"?

Another question is "B1 - Real, Residential, Multi-Family" seems similar or overlapping with "B4 - Real, Residential, Four- or More-Family". I googled around but didn't find a good resource that further explains these categories and what exactly they mean and how they are used.