How to let Fluent recognize diffrent faces

Steven

I am trying to implement a user defined scalar with UDF. There are three regions with different properties in my mesh, and therefore, there will be three different diffusivities for the mass transfer in these three regions. I could not figure out what macros to be used for this purpose, and how to let Fluent recognize the zones in my mesh. The structure could be something like this:

If region==1 diffusivity=value1; else if region==2 diffusivity=value2; else diffusivity=value3;

My questions are:

What macros to be used? Are threads used by UDF to represent different regions in my mesh? What will be the names of the threads? The names of faces in my mesh? Can somebody give me some ideas or hints? I will really appreciate it.

Steven