If you go to a layout for a business object you can edit the toolbars, on the toolbar configuration there is a Sub menu tab:
Click on it and then start adding your sub menu commands, it will open a new button editor and you can configure the entries. A typical usage would be to display groups of quick actions or templates from within a QA category grouping. This allows yo for example to have multiple templates all separated by category e.g. Desktop issues, Server Issues, Printer Issues etc.