By default, cleavage on an aromatic ring is not activated. However, when you dealing with small aromatic compounds, you should activate bond cleavage on aromatic rings by clicking the Cleavage check box in the Allowed on Aromatic Systems pane in the Additional tab in the Reaction Restrictions dialog window. For example, when you generate fragments and mechanisms of phenol, the important fragment corresponding to the peak m/z 66 can be generated only if cleavage on aromatic systems has been activated.

Cleavage on aromatic systems is deactivated by default (Due to the fact that huge numbers of fragments can be generated from large aromatic compounds and the aromatic bond is a very strong bond.)