Which one to choose is mostly a matter of convenience: Certain equations are quicker and easier to specify in one particular form. Internally, equations written in general or coefficient form are converted to weak form, which is the most fundamental form. In particular, the weak form is closely linked to the theory behind the finite element method, FEM. The PDE, Boundary Elements interface uses the boundary element method, BEM, for solving source-free PDEs. In a similar way, the wave form is linked to a discontinuous Galerkin version of FEM, particularly suited for solving wave-propagation problems.