The Nonisothermal Flow multiphysics feature can be added to a model containing a single-phase flow and a heat transfer interface, to realize the two-way coupling between the two interfaces.
It is common to start a model with a single physics (for example, fluid flow), then implement the second one (for example, heat transfer). Then adding the Nonisothermal Flow multiphysics feature realizes the two-way coupling. It also redefines the consistent stabilization so that the multiphysics coupling effects are accounted for in the numerical stabilization.