Typically, a cell is a small 2D drawing, or a 3D object— usually of a frequently-used or complex symbol, notation, detail, or part. Cells are defined as models in a DGN file called a cell library. A cell library can contain many cell definitions each of which is stored in its own model. A cell library is a DGN file. Any DGN file can be attached as a cell library and any cell library can be opened as a DGN file.