When you select polylines to use in wall endcap styles, each polyline corresponds to the shape of the endcap for one wall component. Each polyline is scaled to fit its assigned component.
Wall components are numbered from exterior to interior. Component index 1 corresponds to the outermost exterior component.
A positive value offsets the endcap from the wall start point in the positive X direction.
You can apply the wall endcap style to a wall style (To Apply a Wall Opening Endcap Style to a Wall Style, an opening endcap style (To Create a Wall Opening Endcap Style,) or to individual walls (To Override a Wall Endcap Style on a Wall.)