As far as I know, for getting a unique solution to a PDE we should impose some boundary conditions to the PDE. The number of required auxiliary conditions is determined by the highest order derivative in each independent variable.  My questions are: In any numerical method, especially in finite element method, is it valid:1-) to use more boundary conditions than the highest order derivative in each independent variable. 2-) to use less boundary conditions than the highest order derivative in each independent variable. For example:1-) In this article and in this article, while the highest order derivative of a spatial variable is in third order authors used two boundary conditions for that variable. 2-) In this article while the highest order derivative of a spatial variable is in third order authors used four boundary conditions for that variable. I can increase the number of articles like this. 
What should be the number of boundary conditions of a PDE
pde;finite element;boundary conditions