The Open button in the File Menu is used to read a file and/or load a cell for editing. The button presents a drop-down menu containing the names of the last eight cells opened for editing, plus ``new'' and ``prev'' entries. If prev is clicked, the last opened cell (if any) will be reopened in the main window.
If one holds down Shift while selecting one of cells from the history list, the Cell Placement Control panel will appear with that cell added as the current master. This applies to cell names and not new or prev. This is a quick backdoor for instantiating cells recently edited.
Selecting new with the Shift button held down opens a new cell with a unique name. This can be used for experimentation, or for other purposes. The Save As command can be used to save the contents to a cell with a more descriptive name, if desired.
Otherwise selecting new will use the prompt line to request a file or cell name to open. The internal keyword open is associated with this button. The accelerator actually maps to the new button in the pop-up menu, i.e., the accelerator will cause prompting for the name of a file or cell to open.
The default name used in the prompt of the cell to edit will be one of the following. Each of these sources is tested in order, and the first one that is visible and has a selection will yield the default name.