Boundary condition setting depend on your physics and study interface. I described principles for continuity for fluid dynamic with moving mesh (for expl. Frozen Stator). If you have a static or level phase shift study, it's not needed to make a another setting for temperature or flow continuity. Mr. Muneer Ismael is correct to show heat transfer example, it's not needed to established another BC settings. In my previous comment I talk about another things. I have attached some pictures for that.

Regards

Dear researchers,

Have a nice weekend with your family....

Dear Nazaruddin

Are you insulting me or what, please don't give such type of comment.

The default of COMSOL multiphysics applies the continuity boundary condition at the interface, i.e. you do not have to give any boundary condition at the interface, it will be treated automatically.

Thank you very much Sir, Muneer Ismael. Please tell me where I get continuity boundary condition and how to apply it.

Dear Mubeer Ismael Sir,

Actually, When I applied default continuity condition I show that the boundary selection option is disable i.e. not applicable all the boundaries.

You needed to create identity pair first. For this reason you must choose Form Assembly from geometry and create (or automatically by COMSOL), identity boundary pairs from definition options (contact before two medium). Later, in physics interface add the "Continuity" and choose Identity Pair from created before list. It can be flow continuity for flow mechanics or continuity for Transport of Diluted Species etc.

Thank you very much, Sir Sergejus Borodinas for your kind information.

I use COMSOL Version 5.2, this version can do the continuity BC easily. The note of Sergejus Borodinas is correct, but I think it for older comsol version. However, to show the Form Assembly process, I have prepared the attached video.

Boundary condition setting depend on your physics and study interface. I described principles for continuity for fluid dynamic with moving mesh (for expl. Frozen Stator). If you have a static or level phase shift study, it's not needed to make a another setting for temperature or flow continuity. Mr. Muneer Ismael is correct to show heat transfer example, it's not needed to established another BC settings. In my previous comment I talk about another things. I have attached some pictures for that.

Regards

Thank you very much, Sir Muneer Ismael for your kind information.