The Continuity node, exists as an option at interfaces between parts in an assembly, from the Pairs submenu. The condition is similar to the Material Discontinuity condition that applies to interior boundaries. The difference is that the Continuity should also be applied when the same material exists on both sides of a pair. The advantage of using a pair feature is that the mesh does not need to be conforming on the two sides of the interface (the two parts pf the assembly). This is especially advantageous for the time explicit discontinuous Galerkin method as the time step depends on mesh size and local speed of sound.