Is it acceptable to assign boundary conditions on curves/lines not used in specifying the geometry?
No. If you are assigning boundary conditions on the geometry then you have to assign it on curves/lines used in specifying the geometry. Consider the situation where lines L1 ans l2 form part of the surfaces which define the geometry (in the figure below). Assume that you also have defined a line L3 which connects points 1 and 3. Consider the situation where the vertical side between points 1 and 3 is to be retstrained from moving in the horizontal direction. Then this boundary condition should be specified on lines L1 and L2 but not L3 (as a short cut). the reason is when the finite element mesh is created lines L1 ansd l2 are associated with the nodes that are created. Line L3 is not associated with these nodes. If you attempt to specify the boundary condition on L3 only the nodes at points 1 and 2 will be asssigned that boundary conditions. All the intermediate nodes between points 1 and 3 will not be assigned the boundary condition.
