The cell infills of a curtain wall unit are set in the curtain wall unit style. You can however, create overrides for these infill overrides in cells. For example, you might want a row of normal rectangular panel infills for a first floor facade. In one specific position, however, you want a decorative nested grid. In this case, you would set the row of concrete panels in the curtain wall unit style, and then override the panel at one specific place with the nested grid.
Curtain wall unit infills can consist of simple panels or nested grids. You can change one infill for another, merge cells, or remove frames from an infill altogether.