When modeling it is always recommended to use selections to predefine and give names to several domains or boundaries. Define the selections under the Definitions node. Setting up these selections will for multiphysics modeling avoid adding two incompatible physics to the same domain. If acoustics and structures are coupled this happens at boundaries, where two distinct physics should meet. For, for example, piezoelectric devices the coupling between the electric and mechanical effect happen on the domain level. Operations can also be performed on the selections if the geometry is complex to find intersections, complement sets and so forth.