The menus themselves are displayed using FrameXML-owned frames. Because of this, menu representations generally do not exist in memory unless the menu is open, and only one menu may be open at a time. The function provided to UIDropDownMenu_Initialize is used to create the required menu's entries when that menu is opened. To implement a dropdown menu or list, one typically needs to create a frame inheriting from UIDropDownMenuTemplate, implement an initialization function to populate the menu when needed, and bind the initialization function to the frame using UIDropDownMenu_Initialize.