Non - Isothermal Flow (nitf)

Sir,

I have assigned materials to the domain elements that are required. And, in the step of inspecting the physics part after assigning materials, it is again asking for domain selection. What does that mean?

So for Non-Isothermal Fluid flow (nitf), is it okay of I select the fluid (ie.. water in my case) or should I select solids also. as heat transfer takes place from solid to fluid and viceversa.