This models is description isoparametric rectangular plane stress finite element model.Element parametric models has length "a", width "b", and constant thickness "th" but isoparametric elements models not need
element dimension. Each of the four corners is assumed to posses two degress of freedom displacements "u" and "v" in the "x" and "y" directions, respectively.Thus this element possesses total nodal forces (force pairs of Fx and Fy) and 2*Node total displacements or nodal degrees of freedom (four pairs of u and v)
Also, in the finite element as select formulation method , an important step
is to assume the pertinent displacement interpolation functions, usually of lineer degree serendipidty family.