Regarding DEFINE_VR_RATE and DEFINE_SR_RATE user defined functions in fluent
 

I am pretty new to chemical reactions and incorporating UDF to fluent. For a steam methane reforming reaction ( 3 equations) I have 3 reaction rate equations and it is converted to UDF files.
I am trying both ways DEFINE_VR_RATE and DEFINE_SR_RATE. The catalyst inside was modeled as a porous zone, with defined porosity. The question arises when the user inputs are providing to the fluent reaction window. Even iam defining the rate of reactions by UDF, the Arrhenius rate equation window asks for values of Pre exp factor and activation energy, also temperature exponent. Do the values need again? Does it overwrite the reaction rates which already defines by UDF?
Also is it possible to do the simulation by DEFINE_SR_RATE ? what is the option I need to select inside the reaction window volumetric or wall surface?

Steam methane reforming reaction happen only in a porous region, with no site species development (assume this model).