Some types of objects let you access a sub-object level to edit their component parts. For example, editable meshes have Vertex, Edge, Face, Polygon, and Element sub-object levels. NURBS models can have Surface, Curve, Point, Surface CV, Curve CV, and Import sub-object levels.

Edge sub-objects level chosen while editing a mesh.

You change the active sub-object level using the Modifier Stack display on the Modify panel.