You occasionally want to assign more than one material to a mesh in blender. You might need different textures on the same mesh, and don’t want to break up the mesh into different objects.
Not much of a problem. Go into Edit mode, select the faces that need a different material, then go to the Material tab in the Properties. Notice that you get an additional set of buttons right below the list of materials: Assign, Select and Deselect whenever Edit mode is activated. With those buttons, you can add a material to a selection of faces (Assign) and select or deselect those faces based on their material.
Materials applied in this way will be picked up by Unity3D without a problem.