I don't know how it is confusing. When the headlights are on, the taillights are on. If your headlight switch is in auto mode, the headlights and taillights will come on when it gets dark out. Any other time the headlight bulbs are illuminated without the taillights being on is because of DRLs. DRLs are to increase the visibility of your car during the daytime and are not intended for use at night or any other time the headlights are needed (because headlights also turn on the taillights). If your actual headlights are on and the taillights aren't, there is a problem somewhere.