A curtain wall unit consists of one or more grids, which are the framework for placing design elements in the curtain wall unit. The highest-level grid is the primary grid, which is also the outside edge of the curtain wall unit.
Each grid in a curtain wall unit has one division assignment that determines whether the grid is divided horizontally or vertically. The division also determines the number of cells, their size and their placement.
Within the primary grid, you can define multiple levels of grids. These grids within are called nested grids. Each subdivision of a grid is called a cell.