Attempting to apply a change to a selection only changes one image

If you are attempting to apply a change to a selection of images but the change only effects one of the images there are a couple of possibilities:
# You have *Primary Only* selected. This setting is found at the lower right hand conner of the main window. It is the middle icon with the 1 on it.
# You are attempting to apply a rating, color label or flag from the Metadata pane of the Inspector. Regardless of the state of the *Primary Only* button if you set these adjustments from the Metadata pane then only the primary selection is modified. To apply a rating, color label or flag to a selection use the keyboard shortcuts or the Metadata-> commands.