You can add and customize pull-down menus on the Mac OS menu bar.
You can create and modify menus to display and organize the commands to best match the way you work. Commands are added to a menu from the Commands list in the Customize dialog box. Once commands are added to a menu, you can reposition commands, and organize commands using separators and sub-menus.
Each menu can contain both standard and custom commands. Commands can be grouped with separators and sub-menus. If a menu is longer than the current display resolution in the vertical direction, it is truncated to fit.
When a menu is truncated, two arrows are added to the menu; one is added at the top and another to the bottom. With the arrows, you can scroll through the list of menu items. Sub-menus are indicated on a menu by an arrow that points to the right. When using a sub-menu, it appears to the right of its associated menu item normally. If a sub-menu reaches the edge of the display, additional nested sub-menus will be displayed to the left of the menu item.
Menu items on the Mac OS menu bar are executed by clicking it. Unlike a menu item, you do not need to click a sub-menu to access the assigned menu items. Position the cursor over the sub-menu to expand it and then click the menu item you want to use.
Sub-menus are used to organize and group similar commands together. You create sub-menus in much the same way that you create a menu. Separators can also be inserted to group similar commands without adding an additional navigation level. There is no clear decision that can be made as to when you might use a sub-menu over a separator. Some possible reason why you might consider to use a sub-menu over a separator is the current length of the menu and how frequently a command might be used.