Holy #$@%! Why is this called "Fill"?
Because composites are not true, virtual, laminated walls ... they are faked with a 2D fill.
Or, perhaps the choice of composite wall (fixed width and fill as in composite settings) or standard wall (adjustable width, single fill) is made via the fill parameter of the Wall tool?
One of the forum moderators • AC 26 USA and earlier • macOS Ventura 13.2.1, iMac Pro 10-Core, Radeon Pro Vega 64