Where am I supposed to go to find all the built-in DEM variables by MFIX that could be used in user defined function?

I’ll be writing a udf of the reaction rate for a DEM simulation. But I need to use some particle-related variables like the mass of one single particle at the initial moment and current moment when writing the code of this udf, I guess this kind of variable may be called DES_MASS or something like this, I am not sure if it’s really called this name. So where should I go to look up all this kind of variables predefined in MFIX?

Have a look at the file model/physprop_mod.f

Thanks a lot for your reply!