The No Diffusive Flux boundary condition is the default boundary condition in the Stabilized Convection-Diffusion Equation physics interface and prescribes a vanishing flux due to diffusion across the boundary:

This is the appropriate no flux condition when the relative convective velocity with respect to the boundary velocity is zero. When the fluid velocity at the boundary is not equal to that of the boundary, it is often convenient to prescribe the total flux including convection. In this case, use the No Flux feature instead.