Item definitions, like action bar definitions, cannot be changed at runtime. New item definitions can be added and then used in regions' action bars.

The appearance of an item in a region's action bar can be changed at runtime. Items can be added and deleted from regions' action bars.

Whenever you perform an action on a pull-down, button, choice, or separator, you must name the region on which to operate. Only the item in the specified region is affected. Optionally, you can specify the name of a class of regions, and the action will be performed on each of the members of the class.

None of these changes affect the item's definition. Only the item's appearance in the specified region or class of regions is changed. The action bar templates are not modified either.