Any variable within a defined model can be marked as parameter and will therefore never be replaced by optimization processes until run time initialization. The parameter value will either be replaced by the value defined within the Input & Control Interface or in case of missing initialization by the default value defined in the Analytical Model Interface.
Param T = 1000; # define temperature T as overwriteable
# parameter with the default value 1000
Param Ea = 3.6; # define activation energy as overwriteable
# parameter with the default value 3.6
Param Do = 1.65; # define diffusion coefficient as over-
# writable parameter with the default
# value 1.65
D = Do * exp(Ea/(k*T)); # Arrhenius law using defined
# parameters.