plasticity material properties

Hi ,

i try to solve one structural mechanics problem. In my case, i want to give yield stress level and isotropic tangent modulus properties to my model.i found one similar tutorial (snap hook) in comsol. they used yield stress level and isotropic tangent modulus.they used isotropic tangent modulus as constant value.but they used following equation as yield stress to the model.

S_yield_hook*(1+1e3*flc2hs(1-X/0.0025[m],0.25))


S_yield_hook - is yield stress of hook material

But i could not understand the below term
(1+1e3*flc2hs(1-X/0.0025[m],0.25))

can anybody help me??