Glossary
1 - Sub-object selection selects only part of the object
A sub-object is 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-objects are vertices, edges, faces, polygons, and elements. To access sub-objects, go to the Modifier panel. In the modifier stack display click the plus-sign icon to display an object's hierarchy, and then choose the sub-object level from the hierarchy. At the sub-object level you can select sub-objects, transform the selections, apply modifiers, and so on.
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).