You can apply or customize a composite structure for a wall.
See Assign a Composite Structure to a Wall, Slab, Roof or Shell.
(Remember that the model’s Partial Structure Display settings will affect the display of composite walls.)
See Partial Structure Display.
If the structure of the Wall is a composite, then the Wall’s thickness is defined in Options > Element Attributes > Composites, and equals the sum of the skins’ thicknesses.
You can edit a selected composite by accessing its settings from its context menu:
See Composite Structures Dialog Box.
See also Reference Line of a Composite Wall.