Picks or unpicks individual components from a group of objects.
Choosing
reveals the following section to the left of the Prompt line:

These icons enable you to set the type of objects (the pick mask) that can be picked without opening the option box. If the button is white, that type of object can be picked. If it is the same color as the menu bar, that type of object cannot be picked. Change the status of what can be picked by clicking the icon.