Controls

box	Represents a collection of controls on a custom Ribbon.
button	Represents a button on a custom Ribbon.
buttonGroup	Represents a group of button controls on a Ribbon.
checkBox	Represents a check box control on a Ribbon.
comboBox	Represents a combo box on a custom Ribbon.
contextMenus	Represents a shortcut menu when you right mouse click.
control	Represents a built-in control on a custom Ribbon.
dialogBoxLauncher	Represents the dialog box launcher in the bottom right corner of a group.
dropDown	Represents a list of items that a user can choose from and a list of Ribbon buttons that a user can click.
dynamicMenu	Represents a menu that can be populated with controls at run-time.
editBox	Represents an edit box on a Ribbon.
gallery	Represents a control that displays a menu of RibbonDropDownItem objects and RibbonButton controls.
group	Represents a group of controls on a Ribbon tab.
item	Represents a static gallery, dropDown or comboBox item.
labelControl	Represents a label on a RibbonGroup or RibbonBox.
menu	Represents a menu on a Ribbon tab or on the Microsoft Office Menu.
menuSeparator	Represents a separator control for a menu control.
separator	Represents a separator control for a group on the Ribbon.
splitButton	A Ribbon control that combines a button or a toggle button and a drop-down menu.
toggleButton	Represents a toggle button control on a Ribbon.

Container Controls

These are a specific subset of controls can contain other controls

box
buttonGroup
comboBox
dropDown
menu
splitButton

idQ

<customUI xmlns="http://schemas.microsoft.com ------- 
         xmlns:x="MyNamespace"> 
<button idQ="x:myButton"/>

By using a namespace this allows two different add-ins to add commands to the same custom group.
They need to refer to that custom group by its qualified name