When editing vertically compound walls, use the Assign Layers tool to assign a row to a layer or region. (It assigns the number, material, and function of that row.)
It is more useful to assign layers to regions vertically, rather than horizontally. For example, you might split finish layer 1 into several regions. Then you could assign another finish row to some of those regions and create an alternating pattern, such as brick over concrete.
You should familiarize yourself with the layer functions of compound walls. See Applying a Function to a Layer of a Compound Structure.... Also see Layer Assignment Rules.
To assign wall layers:
All regions currently assigned to that row are highlighted in the preview pane.