Pick Material From Object lets you select a material from an object in the scene. Click the eyedropper button, and then move the eyedropper cursor over the objects in the scene. When the eyedropper cursor is over an object containing a material, it fills with "ink" and a tooltip with the name of the object pops up. Click the object. The material is placed in the active sample slot.
If the eyedropper cursor is over an with faces selected at the sub-object level, and the mesh has a applied to it, then the eyedropper picks up the sub-material. However, if the selected faces have more than one sub-material assigned to them, then the eyedropper picks up the entire Multi/Sub-Object material.