2D selection behaves much like selection in the 3D display (see section 5.1.6). Left clicking will select an individual atom, bond, or residue (if in sequence mode). When viewing a depiction, the right mouse button allows lasso style selection, as in the 3D display. When viewing a sequence, holding down the Shift key when selecting will allow selection of a continuous range of residues.
The current selection is reflected in the coloring of the 2D depiction. There may be slight variations between the depicted selection in 2D and 3D due to the difference in how hydrogens are drawn.
Finally, the 2D display can be toggled to use either aromatic bonds or Kekule bonds via the Molecule Styles tab in the Preferences dialog (see chapter 12).