Each source may have one or more top-level cells specified. If no top-level cells are specified the default operation will be as follows. If the source is a layout file, the entire file will be streamed into the output. If the source is a CHD, the cell hierarchy of the CHD's default cell will be streamed into the output. If not explicitly configured, this will be the first top-level cell in the file referenced by the CHD.
The top-level cell names are names of cells in the source. If the source is a CHD with cell name modification, the names must include the modification. These cells, and possibly their hierarchies, will be used in output. Note that the cells listed are not necessarily top-level in the source, any cell in the source file can be listed.
Cells are added to the list by use of the New Toplevel button in the Options menu. Note that the user must generally know the names of the cells in the source to be extracted. If an empty cell name is given at the prompt, the text ``<default>'' will appear in the listing, which will correspond to the default cell of a CHD source or the first top-level cell found in a source file. Thus, it is possible to access the ``top'' cell in a source without knowing its name. Giving a cell makes available the ``per cell'' operations in the Basic and Advanced pages to the right of the listing.
One can use the Contents list in the Cell Hierarchy Digests listing to list the cell names, if the source has a corresponding CHD. Cell names can be dragged directly from the listing panel and dropped in the Top-Level Cells list, bypassing the need to use the New Toplevel menu button.
Clicking on a cell name in the list will select it, enabling additional ``per-cell'' entries which apply to this cell and its hierarchy. The selected cell name can be deleted from the list with the Remove Toplevel button in the Options menu.