Thin Film BAW Composite Resonator simulation in COMOSL 3.5a

I'm currently engaged in the simulation of FBAR device in COMSOL 3.5a, and want to use this model.

However, "in the model library", I find that the admittance of electrode is defined as "0.5[mm]*nJs_smppn/1[V]", where 0.5[mm] is the width of the electrode, and "nJs_smppn" is the surface current density, which I don't know why, because we use the integration coupling variables, and the width of the electrode is the variable.

Then why we use the "0.5[mm]*nJs_smppn/1[V]", but not the "1.7[mm]*nJs_smppn/1[V]", where 1.7[mm] is the thickness of the 2D scheme of FBAR device.