This tool cannot be used to select grouped objects or the sub-objects of composite paths. Grouped objects or composite paths will function as an integrated object.
Detail of the area selection of the Object Selection tool:
To add additional objects or release the selected object, hold down the Shift key and drag the mouse over the object. In the example, the rectangle is selected. To add the circle to the selection, press the Shift key and drag the mouse over the circle.
With the Object Selection by Path Only option cleared, click any place within the object path to select the object.
With the Object Selection by Path Only option selected, you are only able to click the path or the anchor point to select the object.