A sub-object is an individual member of or a subset of an object's geometry. Many objects have various types of sub-objects that you can work with independently. For example, an editable mesh object's sub-object levels are Vertex, Edge, Face, Polygon, and Element. To access sub-objects, go to the Modifier panel. In the modifier stack display, click the plus-sign button to display an object's hierarchy, and then choose the sub-objects level from the hierarchy. At the sub-objects level you can select sub-objects, transform the selections, apply modifiers, and so on.
Left: A selection of face sub-objects
Middle: A selection of edge sub-objects
Right: A selection of vertex sub-objects
Many topics in the online reference deal with sub-objects. To see a list of sub-object-related topics, search on the term “sub-object” (include the quote marks in the keyword).
Sub-Object ID
Each individual sub-object such as vertex, edge, and face has a unique ID, which appears at the bottom of the Selection rollout on the Modify panel when the sub-object is selected.
The type and ID of a single selected sub-object appear at the bottom of the Selection rollout (highlighting added).
This ID is used by some functions such as Object Paint 
Fill when multiple edges are selected simultaneously.