In axisymmetric components, boundaries on the symmetry axis are boundaries where only a condition for the axial symmetry exists. The COMSOL Multiphysics software adds a default Axial Symmetry node that is active on all boundaries on the symmetry axis. The condition on the symmetry axis is typically a zero Neumann or no-flux condition.

The selection list for boundaries is not available because this is a default boundary condition. In the list, boundaries that are not on the symmetry axis have (not applicable) added after the boundary number.