The divisions of a grid define the orientation that determines the direction of the grid cells and mullions, and they define a division type that determines the number and size of cells.
You can create a variety of division definitions under different names, and then assign different divisions to each grid in your curtain wall. Although you can create multiple definitions, there can be only one division assignment per grid. You create division definitions for a specific curtain wall style and those definitions can be assigned only to grids in curtain walls of that style.