Elements might change their dimensionality when nodes are merged (i.e. a tet might become a tri, a quad a line, etc.).
If you have a 3D mesh you can use this parameter to define if the mesh should still contain 3D elements only after calling this method or if 1D and/or 2D elements are also allowed.
Example: Two nodes of a Tet are collapsed and it becomes a Tri.
If ```min_elem_dim < 3``` this element is integrated into the new mesh, if ```min_elem_dim == 3``` it is not because ```Tri::getDimension()``` is 2.